Gaius Cassius Longinus (Classical Latin: [ˈgaː.i.ʊs ˈkas.si.ʊs ˈlɔŋ.gɪ.nʊs]; October 3, before 85 BC – October 3, 42 BC) was a Roman senator, a leading instigator of the plot to kill Julius Caesar, and the brother in-law of Marcus Junius Brutus. 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.
He was elected Tribune of the Plebs in 49 B.C. He opposed Caesar, and he commanded a fleet against him during Caesar's Civil War: after Caesar defeated Pompey in the Battle of Pharsalus, Caesar overtook Cassius and forced him to surrender. After Caesar's death, Cassius fled to the East, where he amassed an army of twelve legions. He was supported and made Governor by the Senate. Though he and Brutus marched west against the allies of the Second Triumvirate, Cassius was defeated at the Battle of Phillippi and committed suicide.
He followed the teachings of the philosopher Epicurus, although scholars debate whether or not these beliefs affected his political life. Cassius is a main character in Shakespeare's play Julius Caesar that depicts the assassination of Caesar and its aftermath. He is also shown in the lowest circle of Hell in Dante's The Inferno as punishment for killing Caesar.
Little is known of Gaius Cassius' early life, apart from a story that he showed his dislike of despots while still at school, by quarreling with the son of the dictator Sulla. He studied philosophy at Rhodes under Archelaus and became fluent in Greek. He was married to Junia Tertia (Tertulla), who was the daughter of Servilia and thus a half-sister of his co-conspirator Brutus. They had one son, who was born in about 60 BC. In 53 BC he took part in the Battle of Carrhae lost by Marcus Licinius Crassus against the Parthians.