Gaius Cassius Longinus was a Roman Senator and one of the leaders in the plot to assassinate Julius Caesar. As a young man he studied philosophy in Greece and was known for his opposition against despotism. He was the brother-in-law of Marcus Junius Brutus.
Along with his brother-in-law Brutus, Cassius urged his fellow Roman Senators to assassinate Caesar on the Ides of March (March 15), 44 BCE. Cassius also wanted to assassinate Antony at the same time, but was dissuaded by Brutus.
After the assassination of Caesar, Cassius and Brutus (the Liberatores) left Italy and moved to take control of the Eastern provinces (Greece, Macedonia, and Syria). The leaders of the Second Triumvirate, Mark Antony, Marcus Ledidus, and Gaius Octavius then moved to take control of the whole Roman world and avenge the death of Caesar.
At the First Battle of Philippi the forces of Cassius met Antony's forces and were defeated. After his defeat and upon hearing false reports of Brutus' defeat, Cassius committed suicide.