How did the crossing of the Rubicon undermine the power of the Senate, and thus the Republic itself?

When Caesar crossed the Rubicon, he demonstrated that he with the power was the one that ruled. The Senate at the time did not have legions available to force Caesar to give up his legions and obey his edicts. When he defied them. the Senators fled Rome, leaving him undisputed rule of the city. His actions set off another civil war, and would set the pattern for future would be emperors. Generals would take care of their legions, who would in turn fight to put the generals on the throne.