Rubicon was the name of a river flowing in the northern part of Italy and marking its boundary with Gaul. Caesar's crossing of it with his troops was considered an act of aggression and prompted a civil war between his supporters and detractors.
Bouchard, Elsa. "Julius Caesar crosses the Rubicon". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11406, accessed 19 December 2014.]