Crossing the Rubicon is a phrase that simply means passing a point of no return. Those using the saying are simply expressing a feeling that they are now committed to a certain course of action. Although the meaning has been lost or forgotten by many over the years, crossing the Rubicon draws on the history of Roman times for its meaning. It was then that Julius Caesar, who would become Roman dictator and usher in an era of emperors, crossed the Rubicon, a river in northern Italy. It was strictly forbidden in Roman times to cross the river with a standing army.
Geographically, the Rubicon runs from the Apennine Mountains to the Adriatic coast, eventually entering into the sea there. The total length of the shallow river is approximately 49.7 miles (80 kilometers). In Roman times, it separated the Roman province of Cisalpine Gaul from Rome proper. No one had permission to bring a formal army across the river, and doing so generally represented an act of treason, war or possibly both.
Julius Caesar served as governor of Gaul and from there carried out a number of wars and attacks on areas to the north that were generally successful. While those successful campaigns brought him some fame and increased his popularity among many of the common Romans, many of those in the ruling class, as well as those with wealth, resented what Caesar was doing. Therefore, they sought to limit Caesar's influence and arrest him or otherwise stop him from gaining influence.
For his part, Caesar, was not only a shrewd military leader but also a capable politician, and could see what the leaders in Rome were doing. Both sides maneuvered and positioned for a couple of years, with most of the posturing beginning in 51 BC. Then, Caesar decided to mount up and proceed toward Rome in January of 49 BC. Caesar knew well the importance of crossing the Rubicon, telling those marching with him that once they crossed the bridge, the only option was to fight it out.
While Caesar's gamble and campaign toward the heart of Rome would ultimately prove to be successful, crossing the Rubicon would become an idiomatic expression meaning to cross a point of no return. Up until crossing the Rubicon, Caesar did have the option of trying diplomacy, though by that point it appeared there would be no peaceful resolution to the disagreements. Still, Caesar had not, technically, done anything against Rome itself until he had crossed the river Rubicon.