Which Roman general crossed the Rubicon in 49 BC?

Caesar, and the crossing mattered because of a rule rather than a river. The Rubicon was the boundary of Italy proper; a governor entering it with his army was, automatically and by definition, at war with the state. The river itself is small enough that its exact course was lost for centuries.

The famous line is Greek. Suetonius reports him saying let the die be cast - a quotation from the playwright Menander, delivered in Greek to an audience of officers who would have caught the reference, which is a rather more calculated moment than the English version suggests. The alternative to crossing was prosecution and the end of his career, so the throw was less reckless than cornered.

He won the civil war, was made dictator for life, and was killed four years later by men who thought they were restoring the Republic. What they got was thirteen more years of civil war and then Augustus, and the Republic never came back. His name became the word for emperor in a remarkable number of languages - Kaiser, Tsar - so a family name is still, two thousand years later, doing duty as a job title.