the Rubicon

His crossing of the river precipitated Caesar's civil war,[1] which ultimately led to Caesar's becoming dictator for life (dictator perpetuo). Caesar had been appointed to a governorship over a region that ranged from southern Gaul to Illyricum. As his term of governorship ended, the Senate ordered Caesar to disband his army and return to Rome. As it was illegal to bring armies into Italy, the northern border of which was marked by the river Rubicon, his crossing the river under arms was synonymous with insurrection, treason, and a declaration of war on the state. According to some authors, he uttered the phrase alea iacta est ("the die is cast") before crossing.... sources {https://en.m.wikipedia.org/wiki/Crossing_the_Rubicon}