Commanding Julius Caesar disobeys the request from Pompey to return home and disband his army. Instead, he commands his army to build a bridge in order to cross the Rubicon River.
Building Caesar's army starts building the bridge that will help them cross the Rubicon River. This would allow his army to go to Northern Italy.
Crossing Caesar's army is finished building the bridge and crosses it. By doing this, Caesar has committed treason and caused war between him and Pompey. Since Caesar is now in the city of Rome, the soldiers would follow Caesar over the city of Rome.