Why Does Brutus Decide to Join the Conspiracy Against Caesar?

Brutus decides to join the conspiracy against Caesar because he fears that Caesar is becoming too powerful. He is ultimately convinced after reading a forged letter with information convincing him that Caesar is becoming too ambitious.

Brutus and Julius Caesar were good friends until Brutus was recruited to join a conspiracy against Caesar. Cassius is fearful of Caesar's growing power and decides to convince Brutus to join the conspiracy. Since Brutus is friends with Caesar, he is understandably skeptical, but he agrees to think it over. To sway him even more, Cassius places forged letters where Brutus will find them. These letters contain information that ultimately sways Brutus over to the conspirator's side.

The group eventually meets at Brutus' home in order to discuss Caesar's assassination. The attack is carried out when Caesar visits the Senate on March 15th, which is referred to as the Ides of March. Mark Anthony escapes and pretends to befriend the murderers. He asks Brutus if he can speak at Caesar's funeral, and the request is granted. Instead of speaking about Caesar, however, Mark Anthony speaks the words "Friends, Romans, Countrymen, lend me your ears," and he proceeds to turn the citizens against Brutus and Cassius. He bribes the Romans by telling them that Caesar left everyone money in his will. Brutus and Cassius are then chased out of town by the angry mob.