Gaius Cassius Longinus was a Roman senator and general best known as a leading instigator of the plot to assassinate Julius Caesar on 15 March 44 BC. He was the brother-in-law of Brutus, another leader of the conspiracy. He commanded troops with Brutus during the Battle of Philippi against the combined forces of Mark Antony and Octavian, Caesar's former supporters, and committed suicide after being defeated by Mark Antony.