Rubicon, in ancient times, the name of a small river in central Italy dividing the province of Cisalpine Gaul from Italy proper. In 49 B.C., when Julius Caesar was governor of Cisalpine Gaul, the Roman Senate, then allied with Pompey, ordered Caesar to disband his army. After much hesitation Caesar in defiance led his army across the Rubicon into Italy proper. This action led inevitably to war with Pompey. The expression “crossing the Rubicon" means taking an important step from which one cannot turn back. (Caesar's phrase at the time was “the die is cast.") The Fiumicino River, which enters the Adriatic just north of Rimini, is believed to be the river the Romans called the Rubicon.