1864 during the Civil War.  Many of Maycomb’s citizens believe that black people are sub-human.

The trial of Tom Robinson gives Atticus opportunities to teach the children about prejudice and its impact on people both black and white.  Jem, as he matures, begins to understand the ugliness of bigotry as seen inside and outside the courtroom during the trial. 

Why did Atticus defend Tom Robinson when he knew that he could not win the case? This unique man explains it to his children:

"If you shouldn't be defendin' him, then why are you doin' it?"

"For a number of reasons," said Atticus. "The main one is, if I didn't I couldn't hold up my head in town, I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again…Scout, simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."

Integrity and character are two of the traits that Atticus tries to instill in his children.  He must demonstrate these qualities himself.  Atticus commits to treating everyone the same.  He does not believe in violence which often occurs in the story.

As a lawyer, Atticus promises to defend anyone that needs his help.  The judge assigned him to the case, but Atticus would have taken the case anyway. 

The case cannot be won at this time in the south.  Neither women nor black people serve on the juries.  Everywhere a person turns there is bias----against women, blacks, people like the Cunningham and the Ewells.

To definitively answer the question, Atticus defends Tom Robinson because it is the right thing to do.  There will no change in prejudice until everyone receives the same treatment in the court system which Atticus states “is the great leveler of all people.” All people deserve a fair trial and a fair judgment by their peers.

Tom Robinson did get the trial in which everything that needed to be said was stated. One small step was taken: the Cunningham man did not want to vote guilty. The jury stayed  out longer than any other black person's jury.   Atticus knew that even one juror might lead to the next trial where five jurors might vote not guilty.

As Mr. Dolphus Raymond tells Scout in front of the court house:  “…you don’t your pa’s not a run-of-the mill man…”

Atticus's evidence begins with the testimony of Heck Tate, who is the sheriff of Maycomb. Heck was called to the Ewell residence on the night of the alleged attack, and he provides evidence about the nature of Mayella Ewell's injuries. Mr. Tate recalls that Mayella was "bunged up" on the right side of her face and that she had finger marks all the way around her neck.

During Bob Ewell's testimony, Atticus asks him to sign his name. When Bob does so, it is clear that he is left-handed. This is significant because the placement of Mayella's bruises indicates that the person who hit her is left-handed.

When Mayella takes the stand, Atticus tries to treat her gently. However, Mayella is completely unaccustomed to being treated with respect and believes that Atticus is insulting her. At the end of her testimony, Atticus asks her to point out the man who attacked her. Mayella points to Tom, who then stands in the courtroom. Tom's left hand is "fully twelve inches shorter than his right and [hangs] dead at his side. It end[s] in a small shriveled hand." It is nearly impossible that Tom could have inflicted the injuries on Mayella's face, and there is no way he could have left finger marks all the way around her throat.

Based on Bob Ewell's surly and combative personality, as is evident from his testimony, it becomes clear that Mayella's injuries were not inflicted by Tom, who testifies that he stopped by to help her because there wasn't anyone else who would do so. Instead, it seems that when Bob Ewell found his daughter had been entertaining a Black man, he beat her and then blamed it on Tom.

Despite compelling evidence to attest to his innocence, Tom is convicted of the charges against him.

The answer to this question can be found in Chapter 18, which is when Atticus cross examines Mayella Ewell. Part of what he does is to prove that Tom Robinson could not have hit her on the right side of her face, as his left hand had been mangled beyond repair by a cotton gin when he was a boy. This piece of evidence is crucial, as it supports earlier comments that Mayella made that her father had beaten and assaulted her. Of course, when confronted with the truth, Mayella does everything she can to deny it and resist it, which loses her sympathy with the reader. Although she undoubtedly enjoys a pitiful existence, she seizes the opportunity to victimise somebody else just as she is victimised, and causes far more harm to others than she herself receives. However, note the following quote that comes from the final chapter of this book, which could be taken as one quote that presents the general theme of the text:

"When they finally saw him, why he hadn’t done any of those things... Atticus, he was real nice..." His hands were under my chin, pulling up the cover, tucking it around me. "Most people are, Scout, when you finally see them." He turned out the light and went into Jem’s room. He would be there all night, and he would be there when Jem waked up in the morning.

Even though they are talking about Boo Radley, the novel as a whole argues that humans need to "see" each other truly, beyond superficial prejudices and stereotypes, to understand that they can be "real nice." What Atticus does in the court room is to force the jury to "see" Tom Robinson as he really is, and as a man who is innocent. He does this by proving he could not have assaulted Mayella.

Atticus accepted Tom Robinson's case because he knew no one else would want to take the case, and if anyone else did, he wouldn't put on a good case or care whether Tom was convicted. Atticus knew the odds of getting Tom off from the very beginning, but he knew that he had to try. Taking these "baby steps" was the only way that racial bigotry was ever going to change in the South. Atticus knows Tom is innocent, and if Tom was white, he would have been found innocent with the evidence Atticus presented. Atticus at least made the jury deliberate for a while before bringing in a guilty verdict.

In my opinion, Atticus accepts this case because that is the kind of man he is.  It is a case that goes along with his values and so he thinks it would be good to take it.

Atticus is a man who thinks that it is wrong to bully the weak.  He does not like it when his kids torment Boo Radley, for example.  And he does not like the way whites treat blacks in Maycomb.  So I think he is taking the case to try to prevent Tom from being abused due to his race.

This is a great question. There are a few important reasons why Atticus believes he needs to defend Tom Robinson, even if it is not a popular thing to do.

First, Atticus believes that Tom is innocent. In fact, in the trial scene, he cogently lays out the evidence for Tom's innocence. He persuades the reader for certain, even if the racially charged jury disagrees.

Second, Atticus is a man who stands for justice. So, he needs to do what he believes is right. In one instance, he even stands before a mob of people to defend Tom Robinson.

Third, in a conversation with Scout, he states that he has to defend Tom Robinson, because his integrity is on the line. He explains that if he does not defend him, then he would not be able to tell Scout and Jem to do what is right, because he would not be doing what is right. Here is the text:

“If you shouldn’t be defendin‘ him, then why are you doin’ it?”

Throughout the trial, Atticus cross-examines the witnesses and proves Tom's innocence by illustrating that Bob Ewell was Mayella's perpetrator. Initially, Atticus confirms that the majority of Mayella's injuries were to the right side of her face and that Bob Ewell is left-handed, which suggests that he may have been responsible for his daughter's injuries. After cross-examing Mayella, Atticus's questions prove that Mayella and her father are fabricating their stories. Atticus then shows the jury that Tom is handicapped and his left arm is completely useless. In Atticus's closing remarks, he proves Tom's innocence by mentioning the lack of medical evidence, the Ewells' conflicting testimonies, and Tom's obvious handicap. Atticus makes a moving argument that explains Mayella's motivation to blame Tom Robinson by elaborating on the strict social code that forbids interracial relations. He also reminds the jury of Bob's terrible reputation and elaborates on his motivation to conceal his daughter's actions. Despite Atticus's brilliant defense, the jury cannot look past their prejudice and wrongly convicts Tom Robinson of assaulting and raping Mayella.

Tom's innocence is apparent to readers because of logic. Mayella had a black eye on her right side of her face. This would indicate that a left-handed person struck her. In Chapter 17, Atticus shows the jury that Bob Ewell is left-handed. Yet in Chapter 18, Tom stands up and the courtroom notices, "His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I (Scout) could see that it wad no use to him."

In chapter 20 of "To Kill A Mockingbird," Atticus delivers his summation to the jury in the trial of Tom Robinson. He first tells the jury that Tom should not have been tried at all because there was no evidence against Tom. He then tells the jury that the only reason they are there is because of Mayella Ewell's guilt over trying to seduce a black man. Her father beat her up and she couldn't stand the thought of seeing Tom all the time knowing what she had done. He tells the jury that Tom is a "quiet and humble" black man and hasn't done anything wrong but feel sorry for a white woman. He talks to the jury about equality in the court room and finally tells the men on the jury

He ends his summation with, "In the name of God, do your duty."

Atticus's chief reason for mounting a good defense of Tom Robinson rather than participating in a mockery of justice is his own integrity. He knows the system is rigged against black people, but he realizes that he doesn't have to capitulate to its ugliness. As he tells Jem,

The one place where a man ought to get a square deal is in a courtroom, be he any color of the rainbow, but people have a way of carrying their resentments right into a jury box. As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it—whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash.

In the above quote, Atticus expresses two of his personal reasons for trying his hardest to insure Robinson a fair trial, even though in the end he knows Robinson will be convicted. First, Atticus believes a court of law should be colorblind and judge people impartially, regardless of their race. Second, he thinks any white person who takes advantage of blacks people is "trash." In other words, Atticus couldn't live with himself if he didn't do his best for Robinson and treat him as he would a white client.

Atticus also wants to model integrity for his children so that they will grow up to be honorable people.

Toward the beginning of chapter 9, Cecil Jacobs tells the entire playground that Scout's father defends niggers, which upsets and confuses Scout. Later that night, Scout tells Atticus what happened at school and asks him about it. Atticus goes on to explain to Scout that he is defending an African American man named Tom Robinson and admits that there has been some "high talk" around the town about his decision. When Scout asks why Atticus is defending Tom against the community's wishes, Atticus responds by saying,

"For a number of reasons . . . The main one is, if I didn’t I couldn’t hold up my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you or Jem not to do something again." (Lee, 78)

Atticus then encourages Scout to try her best to ignore the negative remarks and refrain from fighting others. Atticus also explains to Scout that even though he knows that he will not win the case, he is going to try his best to defend Tom Robinson. Atticus ends the conversation by telling Scout,

"This time we aren’t fighting the Yankees, we’re fighting our friends. But remember this, no matter how bitter things get, they’re still our friends and this is still our home." (79)

Scout is receptive to her father's explanation and tries her best to control her anger. The next day, Scout demonstrates her self-control by walking away after Cecil Jacobs continues to criticize Atticus.

Atticus has many reasons for deciding to defend Tom Robinson in Harper Lee's novel, To Kill a Mockingbird, but "the main one is, if I didn't I couldn't hold my head up in this town." Scout has nearly found herself fighting again, this time with Cecil Jacobs over his claim that Atticus "defended niggers." So, Scout asks Atticus directly. After scolding her not to use the "N" word--"That's common"--he explains why he has undertaken the unpopular decision to defend Robinson, a black man, at his trial for raping a white woman.

"I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again... I could never ask you to mind me again... every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."

Scout also discovers on the day of the trial that Atticus did not volunteer to defend Tom: He was asked by Judge Taylor himself.

Scout is young and doesn't understand everything her father is telling her--yet.  In fact, Harper Lee often uses Scout's youthful ignorance as an opportunity for the adults to explain what she wants the readers to hear about prejudice and hate and other harmful stereotypes.

In this case, Atticus does explain his position, as you noted.  He tells Scout, in answer to her question, that they will probably not win this case.  He then asks her for one thing--to keep her cool and not pick a fight over this matter.  Her reaction is pretty basic, considering she really doesn't understand what her father is asking her to do--to accept that others will be ignorant and cruel and hurtful because they have not learned better.  She is capable of being the bigger person, and that's what he asks of her. She promises. 

She tries, she really does. but when her cousin Francis calls her father names she simply has to haul off and whack him--prompting Atticus to let Scout overhear a conversation in which he admits his fears for the summer ahead and his hope that his children will come to him first rather than try to fight.  She doesn't make another promise, but she does better the next time she is tempted.

At the beginning of chapter 9, Atticus tells Scout that she needs to learn to maintain her composure when facing adversity, and she proceeds to asks him why Cecil Jacobs said Atticus is defending a black man. Atticus goes on to tell Scout that he is defending a black man named Tom Robinson, which is an unpopular decision among the community members. When Scout asks why he is defending him, Atticus tells her,

For a number of reasons . . . The main one is, if I didn’t I couldn’t hold up my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you or Jem not to do something again (Lee, 78).

In chapter 11, Atticus has another conversation with Scout, where he encourages her to exercise self-control, because people in the community will say awful things about him. When Scout tells her father that he must be wrong for defending Tom Robinson because the majority of the citizens disagree with his decision, Atticus once again explains his reasoning for defending Tom by saying,

This case, Tom Robinson’s case, is something that goes to the essence of a man’s conscience—Scout, I couldn’t go to church and worship God if I didn’t try to help that man . . . They’re certainly entitled to think that, and they’re entitled to full respect for their opinions . . . but before I can live with other folks I’ve got to live with myself. The one thing that doesn’t abide by majority rule is a person’s conscience (Lee, 108).

Essentially, Atticus feels morally obligated to defend Tom Robinson, because he knows that it is the right thing to do, regardless of the community's perception and feelings.

Atticus is a realist, and he has few expectations of actually winning the case. He knows that in 1930s Alabama, a black man accused of raping a white woman has virtually no chance of being acquitted. Tom will face an all-white jury and it 

"... couldn't possibly be expected to take Tom Robinson's word over the Ewells'.  (Chapter 9)

He believes Tom's only chance is on appeal.

"... I intend to jar the jury a bit--I think we'll have a reasonable chance on appeal... "  (Chapter 9)

But Tom's case is not one that Atticus relishes. He did not volunteer to defend Tom; instead, he was handed the case by Judge John Taylor, probably because Taylor recognized that Tom may be innocent, and that Atticus--the best attorney in Maycomb--would be Tom's best chance for a fair trial and of regaining his freedom.

"... I'd hoped to get through life without a case of this kind, but John Taylor pointed to me and said, 'You're It.' "  (Chapter 9)

Atticus chose not to turn down the case because he believes Tom's version of what happened and knows that the Ewells must be lying, but he primarily took it because of Jem and Scout.

"... do you think I could face my children otherwise?"  (Chapter 9)

Atticus does not volunteer to defend Tom Robinson. Instead, he is assigned the case by Judge Taylor, who

"... pointed at me and said, 'You're It.' "

Atticus would have preferred not to have been the judge's choice, and he confided in his brother, Jack, that

"I'd hoped to get through life without a case of this kind..."

But Atticus has another reason for not refusing his assignment: He does it to set an example for his children.

"... do you think I could face my children otherwise?... I hope they trust me enough..."

Atticus recognizes that Tom is innocent, and he is able to see what the jury apparently does not: That Tom could not have committed the crimes of which he was accused because of his crippled left arm. Atticus also understands that the left-handed Bob Ewell must have beaten his own daughter. Atticus has taught his children that the "worst thing" a white man can do is "cheatin' a colored man," and he aims to exonerate Tom from the trumped-up charges made by the Ewells.

Since Atticus Finch has always held to liberal principles, he feels that he must defend Tom Robinson as a matter of upholding his personal as well as his professional beliefs. In other words, he must "practice what he preaches."

It is, indeed, a matter of personal integrity for Atticus to act as the defender for Tom Robinson in his unjust trial for rape. As Atticus explains to his brother Jack in Chapter 9, he feels that he must take on the task in the hope that by doing so, he can at least prevent his children from "catching Maycomb's usual disease." And, as the only lawyer in town who will truly try to uphold the "justice for all" under which legal trials should be conducted, Atticus feels it is his professional obligation to accept the position as defender.

Further, in Chapter 11, Atticus tries to prepare Scout for "worse things" than the insults of Mrs. Dubose as the trial for Tom nears. He hopes that she and Jem make look back on his defense of Tom

"...with some compassion and feeling that I didn't let you down. This case, Tom Robinson's case, is something that goes to the essence of a man's conscience--Scout, I couldn't go to church and worship God if I didn't try to help that man."

With these words to his daughter, Atticus expresses his personal obligation as a Christian to defend Tom Robinson as well as to affirm his personal and professional ethics.

Atticus takes the case because it's the right thing to do.  Atticus is an educated man who believes in fundamental human rights.  He takes the case because he was asked.  He takes the case because he knows no one else would.  He takes the case because he is the kind of man who displays quiet dignity.  He tells Scout that he takes the case because if he didn't he couldn't hold his head up anymore nor could he tell her and Jem what to do anymore.  Atticus Finch takes the case because it's not about winning or losing to him it's about doing right by his conscience and doing right by his children.

In To Kill a Mockingbird, Tom Robinson, a black man, is charged with the rape of a white woman. In Maycomb, this is as good as a death sentence for a black man. Atticus has always been the kind of man who believes in doing the right thing. He believes that all people are created equally, yet he knows that in the community a black man has almost no chance of getting a fair trial. When the judge comes and asks Atticus to defend Tom, Atticus agrees, knowing that he is probably the only chance Tom has. Atticus also knows that because he is defending him, his children will start hearing some bad things, but Atticus believes in doing what is right.

Atticus is a fair and just man. He believes that everyone is entitled to the best defense they can get. Atticus knows that Tom is innocent, but he knows that it will probably be impossible for him to prove that.

"Simply because we were licked a hundred years before we started is no reason for us not to try to win."

Atticus tells Scout that Tom will probably be convicted and Scout asks him why he is defending him if he knows he will be convicted. 

"For a number of reasons," said Atticus. "The main one is, if I didn't I couldn't hold up my head in town, I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again...Scout, simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."

Atticus knows that this case and trial will have a long-lasting affect on not only him, but on his children and the whole town. Yet he knows that he has to do it. Atticus is the kind of man who wouldn't be able to live with himself if turned his back on Tom. Atticus doesn't realize the beautiful legacy he is leaving for his children.

Atticus defends his client, Tom Robinson, against Bob and Mayella Ewell throughout the trial. Bob and his daughter both accuse Tom Robinson of committing assault and rape on the evening of November 21st. Mr. Gilmer is the prosecuting lawyer who opposes Atticus during the case and attempts to portray Tom as a guilty man. Atticus not only defends Tom Robinson against the Ewells and Mr. Gilmer, but he also defends Tom against the prejudiced jury and community. In the small town of Maycomb, Alabama, prejudice against African Americans is commonplace, and Atticus valiantly swears to defend his client against racial injustice. Despite Bob and Mayella's conflicting testimonies, lack of evidence, and Tom's obvious handicap, Atticus loses the case. The racist jury members convict Tom Robinson of assaulting and raping Mayella Ewell simply because he is black. Tom tragically becomes a victim of racial injustice.