𝗧𝗼 𝗰𝗿𝗼𝘀𝘀 𝘁𝗵𝗲 𝗥𝘂𝗯𝗶𝗰𝗼𝗻 means to make a decision, or take a course of action, or decisive step, which once embarked upon cannot be altered, and from which there can be no turning back or opportunity to change one’s mind. The phrase arose from the action of Julius Caesar, in 49 BC, when he was ordered to disband his army. Instead of doing so he led his dedicate legion across the shallow river Rubicon, which bounded his Gallic province in northern Italy, and marched against Rome, exclaming ‘the die is cast’. This brought about what amounted to a declaration of war against the Senate.
Fortunately for Caesar, his decision was successful and he became master of the Roman world.
P.S: To learn all politics through the Romans has evolved me into accepting of all the manipulation in this world and understand people/power better.