Caesar and his army approached Rome and "crossed the Rubicon", a shallow river in north-east Italy, in 49 BC. It was the point beyond which no army was supposed to go. The river marked the boundary between Cisalpine Gaul to the north, and Italy proper to the south. Crossing the Rubicon caused a civil war. Pompey, the lawful Consul, and his friends, fled from Rome as Caesar's army approached.