Cassius have known Caesar as a person for a long time; they know that he is, for example, not a strong swimmer and once required their help to prevent him drowning. For him to set himself up as a god, then, is ludicrous. He must be stopped.

In act one, scene two, Cassius tells Brutus to let him be his "glass," meaning his mirror to better see himself. He then proceeds to interpret, for Brutus, all of Brutus's reactions to what they can hear of Caesar's interaction with the citizens. When Brutus says that he fears that "the people / Choose Caesar for their king," Cassius replies that this must mean that Brutus recognizes that Caesar should not be "their king." Brutus, like Cassius, is concerned that Caesar is becoming too powerful.

Cassius then tells Brutus that they are as worthy as Caesar, that they "both have fed as well" and were "born free as Caesar." The implication that Cassius is trying to push here is that they are as worthy as Caesar and, therefore, Caesar should not be more powerful than they are. Cassius continues to mock Caesar's masculinity or supposed lack thereof. He relates a story in which Caesar was drowning and asked for Cassius's help and then says, incredulously, that this same Caesar "is now become a God." Again, Cassius is trying to persuade Brutus that Caesar has become too powerful and is underserving of this power. To emphasize the point, he calls Caesar a "coward" and compares him to "a sick girl."

As persuasive as Cassius's arguments are up to this point, he really hits a nerve, so to speak, when he questions what Caesar is doing to Rome. Brutus truly loves Rome and eventually agrees to join the conspiracy because he genuinely believes that Caesar must be removed for the benefit of his beloved Rome. Cassius points out that Rome has never been ruled by just one man before:

When could they say till now, that talk'd of Rome,That her wide walls encompass'd but one man?

Indeed, before Caesar, Rome was ruled by three men. Cassius suggests that now Rome is becoming something like a dictatorship. Finally, at the end of their conversation and just before Caesar re-enters, Cassius invokes the name of Brutus's relation, who, long ago, once fought to save Rome:

There was a Brutus once that would have brook'dThe eternal devil to keep his state in Rome

In summary, Cassius persuades Brutus to join the conspiracy against Caesar firstly by highlighting how much power Casear has accrued and then by questioning his right to this power given that he is no better than them. Cassius then says that Caesar is in fact less than them—he is cowardly and weak. He then suggests that Caesar is endangering Rome, which Brutus loves and is loyal to, and finally he implies that it is also, for Brutus, a question of family honor to protect Rome.

In Shakespeare's Julius Caesar, Brutus is depicted as a noble, honorable man who is manipulated by the selfish Cassius to join the group of senators conspiring to assassinate Julius Caesar. At the beginning of the play, Brutus is reluctant to act on his impulses and take extreme measures. Brutus recognizes that Julius Caesar is a widely popular figure who is only gaining more power in Rome. Despite his concerns regarding Caesar's rising popularity, Brutus stills views him as an even-tempered man with good intentions. However, Cassius cleverly plants the seed of suspicion in Brutus's mind, and Brutus begins contemplating Caesar's ambitious nature.

In act 2, scene 1, Brutus gives a passionate soliloquy and describes his thoughts regarding his decision to assassinate Caesar. Brutus says,

But ’tis a common proof

That lowliness is young ambition’s ladder,

Whereto the climber upward turns his face.

But when he once attains the upmost round,

He then unto the ladder turns his back,

Looks in the clouds, scorning the base degrees

By which he did ascend. So Caesar may.

According to Brutus's soliloquy, he genuinely believes that Caesar is a threat to the Roman Republic. Having ascended in power, he will quickly turn tyrant when given the chance. Cassius has successfully manipulated Brutus into viewing Caesar as an ambitious politician who is determined to disband the senate and become Emperor of Rome. Following Caesar's assassination, Brutus addresses the Roman people and explains his intentions in act 3, scene 2:

If then that friend demand why Brutus rose against Caesar, this is my answer: not that I loved Caesar less, but that I loved Rome more. Had you rather Caesar were living and die all slaves, than that Caesar were dead, to live all free men? As Caesar loved me, I weep for him. As he was fortunate, I rejoice at it. As he was valiant, I honor him. But, as he was ambitious, I slew him.

Brutus kills Caesar because he believes doing so will defend Rome against tyranny; he has been convinced that Caesar's ambition was a significant threat to the Roman Republic. He feared that Caesar would tyrannize the population, and he wanted to protect the Roman populace.

Cassius feels that Brutus would be a useful member of the conspiracy, for more than one reason. First, he is close to Brutus, and feels he will be able to convince him of the value of the idea. He plays upon Brutus' love for Rome, and the ideals of Rome, to convince him that Caesar's behavior is wrong and must be stopped -- something he thinks Brutus already believes. Cassius reminds Brutus that Caesar is just a man, the same as they are, and they should not allow him to set himself up as a king.

Cassius's other reasons for choosing Brutus specifically revolve around Brutus's standing in Rome, with the senate, and with Caesar himself. Brutus is Caesar's "angel," beloved of him, whereas Caesar dislikes Cassius and would therefore be distrustful of any instruction Cassius might give him. Cassius needs Brutus on hand because Brutus will not only know Caesar's movements, but be able to persuade him one way or another to be at the senate on a certain day.

Cassius is portrayed as a shrewd politician who is willing to go to great lengths to enact his political agenda, and he attempts to convince Brutus to join the group of conspirators against Julius Caesar. Cassius wants Brutus to join the conspirators for several significant reasons. Cassius is aware that Brutus is an honorable man who is respected and revered by the Roman populace. Having a man of Brutus's caliber and rank on their side gives Cassius and the conspirators much-needed support from the Roman citizens. Cassius is aware that Brutus's honorable reputation will justify their actions to the Roman populace, and the citizens will be less likely to riot against the senators. Cassius also knows that Brutus is a close friend of Julius Caesar. If Brutus joins the conspirators, Cassius will now have an inside man who will be able to persuade and influence Caesar into walking into their trap. Brutus will also be able to relay valuable information to the conspirators that will assure that their assassination attempt is successful.

Brutus decides to join the conspiracy against Caesar after his realization to the fact that the Roman Republican government was in great danger.  Caesar had himself declared himself dictator for life. (the ancient Roman position of dictator was defined as a necessary governmeantal position in times of crisis, not to be confused with the modern definition of dictator) However, Caesar's power grew ever stronger and Brutus saw this as tyrannical.The Senate to which Brutus was a member became nothing more than a puppet on the hand of Caesar.The love Brutus had towards the Republican principle and the sense of duty to secure that principle ultimately forces him to choose between a man he greatly admired and the political philosophy which was to serve the greater good.  In his mind Brutus was doing what was "noble" and morally right, in this manner the assassination of Caesar had moral and political justification.

In deciding whether to join the conspiracy, Brutus must choose between his friendship with (and admiration of) Caesar and his public responsibility to prevent Caesar’s alleged ambition to undermine the Roman Republic. Cassius has convinced Brutus that Caesar plans to install himself as monarch. A monarchy would rob the Romans of their long-held liberties.

In his soliloquy in Act II, Scene i, Brutus resolves his inner conflict. He states that he will join the conspiracy because of Caesar’s ambition to be crowned:

It must be by his death: and for my partI know no personal cause to spurn at him,But for the general good. He would be crown’d:How that might change his nature. . . .

. . . . . . . . . . . . . . . . . . . . . . . .

And therefore think him as a serpent’s egg Which, hatch’d, would, as his kind, grow mischievous, And kill him in the shell.

After Caesar’s assassination, Brutus explains his actions to the plebeians (average citizens) in the Forum (Act III, Scene ii):

. . . If then that friend demand why Brutus rose against Caesar, this is my answer: Not that I loved Caesar less, but that I loved Rome more. Had you rather Caesar were living and die all slaves, than that Caesar were dead, to live all free men? As Caesar loved me, I weep for him; as he was fortunate, I rejoice at it; as he was valiant, I honor him: but, as he was ambitious, I slew him.

Brutus offers the audience insight into what motivates him to join the conspirators in assassinating Julius Caesar in act 2, scene 1. When Lucius exits, Brutus contemplates murdering Caesar and begins by mentioning that he has no personal grievances against Caesar. Brutus then wonders how attaining ultimate authority might alter Caesar's behavior. Brutus also questions Caesar's ambition and admits that he has never witnessed Caesar's emotions get the better of him. Brutus then reveals the reason he will join the conspirators by saying,

Fashion it thus: that what he is, augmented, Would run to these and these extremities. And therefore think him as a serpent’s egg—Which, hatched, would as his kind grow mischievous—And kill him in the shell (Shakespeare, 2.1.30-34).

Essentially, Brutus is saying that in order to prevent Caesar from ruling Rome as a tyrant, he must kill Caesar before Caesar gets the opportunity to rule as a tyrant and allows his ambition to negatively affect his behavior.

Brutus also justifies assassinating Caesar just before Mark Antony gives his funeral oration. Brutus tells the masses that in killing Caesar, he was doing the Roman populace a favor by preventing the rise of a future tyranny. Brutus reveals his honorable intentions by telling the crowd,

With this I depart: that, as I slew my best lover for the good of Rome, I have the same dagger for myself when it shall please my country to need my death (Shakespeare, 3.2.42-44).

Brutus justifies killing Caesar on the grounds that Caesar would become a king. In his soliloquy in the opening scene of Act II, Brutus describes his former friend as a "serpent's egg/ Which hatch'd would as his kind grow mischievous." Brutus says in the same soliloquy that he has no "personal cause" to kill Caesar. After the deed is done, Brutus observes, "ambition's debt is paid," a theme he repeats in his speech to the people of Rome. He says Caesar was his friend, but that "as he was ambitious, I slew him." Essentially, Brutus fears Caesar will, by virtue of his exploits and popularity, name himself king of Rome, overthrowing the Republic Brutus feels honor-bound to protect. Caesar was Brutus's close friend, but he was a threat to Rome and Roman liberties, so Brutus killed him. Rome, he assures the crowd, comes before his personal feelings. Some of the assassins had ulterior motives, but Brutus acted from a sense of patriotic duty.

Brutus found the letter Cassius had Cinna leave for him, and assumed the people of Rome were pleading with him to act against Caesar. The letter convinced him there was popular support for his joining the conspiracy. He pondered it before his guests, the other conspirators, arrived.

'Speak, strike, redress!' Am I entreatedTo speak and strike? O Rome, I make thee promise:If the redress will follow, thou receivestThy full petition at the hand of Brutus! (Act II, Scene 1)

Brutus cared deeply about public opinion and his reputation. This was why he was easily persuaded to join the conspiracy. Cassius arranging the letters just made it easier for him to believe he was important, and people were talking about him as a leader. Brutus already had an exaggerated idea of his own importance. The letter played on that idea and convinced him to take the plunge.

Once Brutus joins the conspiracy, he does so at full-tilt. He takes complete control, not allowing Cassius any say in how things happen and not taking any advice from him. Brutus is convinced he is the one the people want, and he knows best.

Brutus's soliloquy occurs in his orchard in the first scene of Act 2.

It must be by his death: and for my part,I know no personal cause to spurn at him,But for the general. He would be crown'd:How that might change his nature, there's the question.It is the bright day that brings forth the adder;And that craves wary walking. Crown him?--that;--And then, I grant, we put a sting in him,That at his will he may do danger with.The abuse of greatness is, when it disjoinsRemorse from power: and, to speak truth of Caesar,I have not known when his affections sway'dMore than his reason. But 'tis a common proof,That lowliness is young ambition's ladder,Whereto the climber-upward turns his face;But when he once attains the upmost round.He then unto the ladder turns his back,Looks in the clouds, scorning the base degreesBy which he did ascend. So Caesar may.Then, lest he may, prevent. And, since the quarrelWill bear no colour for the thing he is,Fashion it thus; that what he is, augmented,Would run to these and these extremities:And therefore think him as a serpent's eggWhich, hatch'd, would, as his kind, grow mischievous,And kill him in the shell.

He is being pressured to join in the plot to assassinate Julius Caesar, mainly by Cassius but also by other Roman aristocrats who fear that Caesar would seriously weaken their powers and privileges if he became king. Brutus is an extremely intellectual man. He will not act impulsively based on feelings but, very much like Hamlet, has to think thoroughly and carefully before undertaking any serious action. He has a strong sense of responsibility. He knows that if he agrees to join Cassius and the others in their plot, then they will move forward because he will have given them the justification they need for their violence. On the other hand, if he decides not to involve himself, the plot may come to nothing. Cassius knows this. That is why he is trying so urgently to persuade Brutus to join him, both by direct personal persuasion and by trickery.

In his soliloquy Brutus tells himself that Caesar intends to be crowned king. He says "He would be crown'd." Brutus is a good friend of Caesar's and knows him better than most people. He is concerned about how Caesar might change if he became the absolute ruler of Rome. Caesar has behaved modestly up to now--almost too much so. But Brutus knows that "lowliness is young ambition's ladder." That is a wise observation. Brutus is thinking that Caesar is only pretending to be humble and modest in order to make an impression on the masses. Brutus strongly suspects that Caesar is wildly ambitious and would become a tyrant if he obtained a monopoly on political, economic and military power.

Brutus decides that Caesar must be stopped because of what he could become if he were made king. Brutus is actually coming to the same conclusion as Cassius, who is much more practical, cunning, and worldly wise than Brutus. The key words in the soliloquy are:

So Caesar may.Then, lest he may, prevent.

In other words, Caesar may become a tyrant, and he should be killed, not for anything he has done, but to prevent him from doing what he might do if he had the power. If they waited until he actually had that power, it would be too late.

Of course, no one will ever know what Julius Caesar actually would have done if he had lived.

When the play opens, it is only Cassius who shows how much he wants Brutus to become a part of the conspiracy he is trying to organize. Without Cassius there might never have been a conspiracy. He knows how to do such things, and Brutus doesn't. Cassius needs Brutus because he knows that he is not popular with either the common people or the patricians. He is not liked because of his character and personality. He is a miser. He is selfish and greedy. He has a violent temper and uses it to intimidate people and to get his way. He is privately thinking that he can be the real power in the conspiracy and use the mild-mannered, scholarly Brutus as a figurehead. But it doesn't work out that way. Once Brutus decides to become involved, he takes command and hardly listens to Cassius' advice.

Cassius thinks they should kill Mark Antony along with Caesar, but Brutus is too noble and too trusting to agree. Then Cassius is horrified when Brutus permits Antony to speak at Caesar's funeral, but Brutus overrules him. Before the Battle at Philippi, Cassius and Brutus have a violent argument in which Cassius threatens to kill Brutus. But Brutus is immune to the kind of scare tactics Cassius uses successfully on other people. After the quarrel ends, Brutus has obviously asserted his dominance, and Cassius just has to go along with him.

The other members of the conspiracy show their respect for Brutus, but they are not instrumental in recruiting him to their cause. It must have been Cassius who recruited them by telling them that Brutus had agreed to lend his name to their cause. They are all glad to have Brutus as their leader because of his distinguished reputation as well as because he is a descendant of the Brutus who drove the Tarquins out of Rome and established the republic. They are all apprehensive about how the masses will react after they assassinate Caesar, because Caesar has made himself so popular with the lower classes. They fear that what could happen is exactly what does happen after Antony delivers his powerful funeral oration. There is mass rioting. Brutus and Cassius have to flee from Rome, along with the conspirators who are still alive.

Brutus is partially persuaded to join the conspiracy because he thinks there are many Romans beseeching him to do so. Actually, the cunning Cassius tricks him. After the two men separate at the end of Act I, Scene 2, Cassius says to himself:

I will this night,In several hands, in at his windows throw,As if they came from several citizens,Writings, all tending to the great opinionThat Rome holds of his name, wherein obscurelyCaesar's ambition shall be glanced at.And after this let Caesar seat him sure;For we will shake him, or worse days endure.

Brutus is made to believe that there are many important citizens who want him to lead a conspiracy to assassinate Julius Caesar. Actually, most of them are elated after the fact. Cassius is a "user." He manipulates Brutus into joining his purely theoretical conspiracy by making Brutus think it is much bigger than it is, and then he recruits others into the conspiracy by showing them that Brutus is the leader. If Cassius is not a villain, he comes very close to meriting that title.

Brutus thinks that once Caesar is given power, he will become corrupt.  To prevent him from bringing harm to Rome, Brutus is wrestling with the idea that he should kill him.

Brutus actually has no specific evidence to think that Caesar will become a bad ruler.  He says,

"...for my part I know no personal cause to spurn at him" (II,i,11-12),

yet speaking generally, he is afraid that power

"...might change his nature, there's the question" (II,i,13).

Brutus believes that if Caesar is crowned, that will put in his hands the capability

"That at his will he may do danger with...Th' abuse of greatness is when it disjoins remorse from power...And to speak truth of Caesar, I have not known when his affections swayed more than his reason" (II,i,16-21).

Because of Caesar's personality, and in particular his tendency to be completely unemotional while relying solely on reason, Brutus feels that there is a great possiblity that he will let power go to his head and do damage to Rome.  Remembering situations where those who have climbed the ladder of ambition have turned their backs on their more noble inclinations once they have achieved their lofty positions, Brutus reflects,

"So Caesar may.  Then, lest he may, prevent" (II,i,27-28).

To protect Rome from what Caesar might do once he is in power, Brutus resolves to kill him, like "a serpent's egg" (II,i,32) while still "in the shell" (II,i,34).

Well, ominously, he starts without giving any reasons, but with a firm conclusion:

It must be by his death

The rest of the speech then works to justify that conclusion - that Caesar's death is the only way to solve the problem in hand. But here's Brutus' argument:

He would be crown'd: How that might change his nature, there's the question.

Caesar wants the crown. And that might change the way he behaves.

... 'tis a common proof That lowliness is young ambition's ladder, Whereto the climber-upward turns his face; But when he once attains the upmost round, He then unto the ladder turns his back, Looks in the clouds, scorning the base degrees By which he did ascend. So Caesar may; Then, lest he may, prevent.

Humbleness and lowliness is like a ladder, which "Ambition" climbs up. When ambition has got what it wants (and got to the top of the ladder), it forgets how it got there, and, rather than look at the ladder, it looks into the clouds. And Ambition "scorns" the way it got where it's got. Caesar could do the same. And therefore, prevent that happening.

...think him as a serpent's egg Which hatch'd would as his kind grow mischievous, And kill him in the shell.

Caesar, therefore, is not dangerous at the moment. But he's like a serpent's egg: one day he will become dangerous, because that's what serpents all are. So better to kill Caesar now before he becomes a full-grown danger.

I don't think it's the world's best argument. But I hope it helps!

Caesar had a high regard for Brutus's reputation for honesty and for his scholarship. Cassius despised Caesar and began to plot against him. Brutus took a great deal of convincing, but he eventually agreed to become a leader of the conspiracy, and it was in fact Brutus who gave a speech to the people immediately after Caesar was killed.

In Julius Caesar by William Shakespeare, Cassius believes that he must have Brutus as a member of the conspirators.  Brutus had a reputation of honesty and persuasive skills that Cassius knew would be needed one the assassination took place. In addition, Brutus was known for his logical, reasoning and strong oratorical skills. 

Cassius believes that he must have Brutus as a member of the conspirators.  Brutus had a reputation of honesty and persuasive skills that Cassius knew would be needed one the assassination took place. In addition, Brutus was known for his logical, reasoning and strong oratorical skills. 

In Act II, Scene I, Brutus is at war with himself.  He does not know what he should do.  In the past, he has been close to Caesar.  Now, he fears the power that Caesar will be given by the senate to him one the Ides of March. 

Brutus speaks in a soliloquy trying to establish what he believes. He knows that later in the morning Cassius will be there to again try to enlist him to join the conspiracy. 

The first issue that Brutus raises is that he has decided that Caesar has to die. On the other hand, he has nothing against him personally.  His death is necessary for the good of the people.  He wants to be crowned the king of Rome.  Brutus wonders how that would change Caesar.

Using the image of a serpent or deadly venomous snake, a person must be careful when around one.  If Caesar is crowned, would he become like the snake and have a sting that would endanger the good of Rome.

Crown him? That; And then, I grant, we put a sting in him  That at his will he may do danger with.  The abuse of greatness is when it disjoins Remorse from power...

When men become powerful, often they lose they common touch; however, Brutus has never seen Caesar show anything but love for the people of Rome. 

Another analogy by Brutus compares Caesar to someone who is climbing the ladder of success.  The ambitious person who is going up the ladder keeps his eye on the goal; yet, when he achieves the highest level, the person often forget those who helped and only focuses on his own achievements.  Caesar may do this, so he needs to be eliminated. 

Referring again to the serpent analogy, Brutus extended the argument. 

Think of Caesar as a serpent egg.  In the nest, it is just an egg.  If the snake hatches, then it becomes deadly and could be harmful.  The best thing to do is to kill the snake while it is still in the egg and avoid the possibility of its sting; so that is what the Romans should do with Caesar.  Kill him before he misuses his power.

Brutus further explains that he has not slept since Cassius first asked him to listen to his opinions about Caesar.  He has been haunted by what he should do.

When Cassius and the other conspirators come to speak to him, Brutus immediately becomes a part of the conspiracy.  The beginning of the end of normalcy for Rome and its citizens has begun. 

Accessteacher is correct, it is in Act II, scene i that Brutus makes his final decision to join the conspirators.  However, it is in his meeting with Cassius in Act I, scene ii that he begins to justify the action of murdering Caesar.

He hints at his own worries over the potential crowning of Caesar around line 40:

...Vexed I am

Of late with passions of some difference...

Which give some soil perhaps to my behaviours.

And then, when the crowd cheers offstage:

What means this shouting?  I do fear the people

Choose Caesar for their king.

And it is the becoming King in the Roman Republic that Brutus will not tolerate.  His justification for killing Caesar is already forming.  He says to Cassius:

Set honor in one eye and death i' the other

And I will look on both indifferently.

For let the gods so speed me as I love

The name of honor more than I fear death.

So, he is willing to commit treason for the honor of holding up the true government of the Republic against tyranny, against a king.

And so he leaves Cassius, who now holds the firm hope that Brutus will join him in murdering Caesar for the good of Rome.  When we next see Brutus in Act II, he has made his decision, the decision that he has alluded to in Act I.

You have asked a question that goes to the heart of one of the themes of the play - the power of rhetoric and manipulation. The speech you want to examine very closely is Brutus' soliloquy in Act II scene 1, where Brutus tries to persuade himself into being involved in the conspiracy. If you were directing this play yourself, you need to think about whether Brutus is actually getting involved for the noble motives he possesses, or whether this speech is actually ironic: in talking about the danger of ambition and how this will probably lead Caesar to become despotic, Brutus is blind to his own ambition and how the same dangers await him.

Brutus starts off my saying there is no personal reason for him to kill Caesar:

It must be by his death; and for my part,

I know no personal cause to spurn at him,

But for the general.

The general good can be the only motive for assassinating Caesar, Brutus assures himself, before moving on to the crux of the issue:

He would be crowned.

How that might change his nature, there's the question.

It is the bright day that brings forth the adder,

And that craves wary walking. Crown him that,

And then I grant we put a sting in him

That at his will he may do danger with.

Note here how Brutus uses a metaphor of an adder to compare what might happen to Caesar if he is crowned. Crowning Caesar would give him more power, giving him a "sting" that he could do serious damage with to democracy.

Although Brutus acknowledges that Caesar has shown himself to be worthy of the power he has received up to now, Brutus falls back on aphorisms and "common proof" to persuade him of the danger were Caesar's power to grow bigger:

But 'tis a common proof

That lowliness is young ambition's ladder,

Whereto the climber upward turns his face;

But when he once attains the upmost round,

He then unto the ladder turns his back,

Looks in teh clouds, scorning the base degrees

By which he did ascend. So Caesar may;

Brutus thus uses such "common proof" to persuade him of the danger, though the use of words such as "may" of course perhaps makes us think that Brutus' conclusion is by no means certain, thus suggesting that other motives come into play that perhaps Brutus is blind to, such as his own ambition and envy of Caesar. Either way, by the end of the speech, Brutus has convinced himself:

And therefore think him as a serpent's egg

Which hatched, would as his kind grow mischievous,

And kill him in the shell.

Brutus and his conspirators must "kill" Caesar now before he "hatches" into the serpent that, according to Brutus, he will obviously become due to the amount of power he is gaining. His choice is made and Brutus has chosen his destiny.