All south of the Rubicon was territory reserved for the immediate jurisdiction of the city.

The Romans, in order to protect themselves from any danger which might threaten their own liberties from the immense armies which they raised for the conquest of foreign nations, had imposed on every side very strict limitations and restrictions in respect to the approach of these armies to the Capitol.

The Rubicon was the limit on this northern side.
Generals commanding in Gaul were never to pass it.

To cross the Rubicon with an army on the way to Rome was rebellion and treason.

Hence the Rubicon became, as it were, the visible sign and symbol of civil restriction to military power.
[Sidenote: Caesar's expenditure of money at Rome.] [Sidenote: His influence.] As Caesar found the time of his service in Gaul drawing toward a conclusion, he turned his thoughts more and more toward Rome, endeavoring to strengthen his interest there by every means in his power, and to circumvent and thwart the designs of Pompey.