Julius Caesar was the famous general who crossed the Rubicon River in 49 BC. This act was significant because it was a declaration of war against the Roman Senate and resulted in a significant shift in power dynamics within Rome. By crossing the river, Caesar effectively initiated a civil war, defying the Senate’s authority and prompting the famous phrase "the die is cast." This decision ultimately led to Caesar's rise as dictator and paved the way for the transformation of the Roman Republic into the Roman Empire.