Gaius Julius Caesar (IPA: ;Official name after 42 BC, Imperator Gaius Iulius Caesar Divus (in inscriptions IMP•C•IVLIVS•CAESAR•DIVVS), in English, "Imperator Gaius Julius Caesar, the deified one". Also in inscriptions, Gaius Iulius Gaii Filius Gaii Nepos Caesar, in English, "Gaius Julius Caesar, son of Gaius, grandson of Gaius".), July 12, 100 BC – March 15, 44 BC) was a Roman military and political leader. He played a critical role in the transformation of the Roman Republic into the Roman Empire. His conquest of Gaul extended the Roman world all the way to the Atlantic Ocean, and he was also responsible for the first Roman invasion of Britannia in 55 BC. Caesar is widely considered to be one of the foremost military geniuses of all time, as well as a brilliant politician and one of the ancient world's strongest leaders. In 42 BC, two years after his assassination, the Roman Senate officially sanctified him as one of the Roman deities.

Caesar fought in a civil war that left him undisputed master of the Roman world, and after assuming control of the government began extensive reforms of Roman society and government. He was proclaimed dictator for life, and he heavily centralized the bureaucracy of the Republic. Ironically, this forced the hand of a friend of Caesar, Marcus Junius Brutus, who then conspired with others to murder the great dictator and restore the Republic. This dramatic assassination on the Ides of March (March 15th) in 44 BC sparked a new civil war in Rome.

Caesar's military campaigns are known in detail from his own written Commentaries (Commentarii), and many details of his life are recorded by later historians such as Appian, Suetonius, Plutarch, Cassius Dio, and Strabo. Other information can be gleaned from other contemporary sources, such as the letters and speeches of Caesar's political rival Cicero, the poetry of Catullus, and the writings of the historian Sallust.