On this day, the 10th of January 49BC, a die was cast by none other than Julius Caesar. Metaphorically speaking, of course. Physically, he and his Legio XIII crossed the Rubicon, the river border between Italy and Cisalpine Gaul in a march to straight to Rome.
Understandably, the Roman Senate were just slightly perturbed by this. They had specific laws against this exact action, the imperium law, making it illegal for any Roman army raised outside of Italy entrance within. It had to be disbanded by the general before entrance would be granted.
Julius, being his usual ambitious self, directly transgressed this law, and intentionally began the Roman Civil War. His actions were the result of years long political strife against his rival Pompey, in which neither could agree on non-violent diplomacy.
Despite Caesar possessing only the one legion, and technically being outnumbered about 10 to 1, Italy was not prepared for such sudden incursion. Pompey and the Senate were forced to flee Rome.
Eventually, after 4 or so years, Caesar was victorious and thus proclaimed dictator of Rome. The Roman Civil war was one of the last military actions before the Republic became the Empire.
Now as to whether or not Caesar actually said "alea iacta est" when he crossed over is up for debate. Whether it was in Latin, or Greek, or even said at all, one thing's for sure;
It would have been pretty cool to say.