There is a relatively unknown incident in history that set Rome off on the path from Republic to Empire. This was a terrorist, stateless, attack on the Roman commercial center of the port at Ostia. The port was burned down, part of the navy was destroyed, two Senators were captured as hostages. After this attack the Senate launched a war on terror and disregarded the system of checks and balances in their constitution. The official proclamation of the end of the Republic came 40 years later but the constitution did not apply in the intervening years.
It may be that the Roman republic was doomed in any case. But the disproportionate reaction to the raid on Ostia unquestionably hastened the process, weakening the restraints on military adventurism and corrupting the political process. It was to be more than 1,800 years before anything remotely comparable to Rome's democracy - imperfect though it was - rose again.This may be an exaggeration of the effect of the pirate raid as the Roman Republic was pretty unstable in its politics and wars at the time. The raid itself may have been launched because of the civil wars and brief dictatorships Rome had been experiencing. On the other hand, 2000 years from now how will history judge and date the beginning of the American Empire and the fall of the American Republic?
A side effect of the military adventurism Rome commenced after the pirate raid was Judea becoming a client-state of the Roman Republic/Empire.