In Shakespeare's Julius Caesar, when the play opens Caesar has just recently defeated another Roman general in a civil war. The Roman crowd hails him as a hero and Caesar is actually offered a crown three times, although he rejects it. This is significant because Rome is a republic, ruled by the Senate. To offer Caesar a crown is to ask him to rule as emperor.
Although Caesar rejects the crown, senators worry that he will not always reject the crown, and he may one day seize power as emperor. They fear his ambition, or at least Brutus, Caesar's loyal friend, does. The other senators appear simply to be envious and jealous of Caesar.
These events and attitudes serve as the catalyst for the play, and lead to Caesar's assassination and a second civil war between Antony and his followers and Brutus and his.
This play is based on historical events. In the beginning of the play, we learn that Julius Caesar has become the most powerful man in Rome. Rome at this time is a Republic. The Roman Senate fears that Caesar is getting too powerful, getting puffed-up with his own power, and that he might make himself dictator or emperor. If this happens, Rome will not be a Republic anymore.
There is a plot being hatched at the beginning of the play, led by a senator, Cassius. The senators are planning to kill Caesar. In fact, Caesar has been warned of this by a soothsayer who tells him, "Beware the Ides of March." Caesar brushes off this warning, however.
Read about the play here on enotes.