A group of 40-60 conspirators--led by Marcus Brutus and Cassius Longinus--assassinated Julius Caesar on March 15, 44 BC--the Ides of March. Brutus and the conspirators chose to murder Julius Caesar because they believed he was planning to overthrow the Roman Senate and institute a tyranny. By murdering Caesar, who had been declared dictator perpetuo--dictator for life--they believed that they could restore the Roman Republic.
Unfortunately for the conspirators, events did not transpire as they had hoped. They successfully completed the assassination, but Marc Antony shrewdly lessened its impact by affirming the validity of Caesar's accomplishments, while also convincing the Senate to not punish the conspirators (preventing them from dying as martyrs). Nevertheless, Rome quickly descended into civil war, as Marc Antony, Brutus and Cassius, and Octavian fought for control of Rome. Ultimately, Octavian prevailed and declared himself the first Roman emperor. Thus, ironically, the actions which the conspirators took to preserve the Roman Republic actually hastened its demise.