Complex sentence: For the first time since the days of Hannibal, Italia and Rome itself were seriously threatened, a threat which led to fundamental reforms of the Roman Army and state, and which would have a significant impact on the course of history.
Split into simpler sentences: For the first time since the days of Hannibal, Italia and Rome itself were seriously threatened. A threat which led to fundemental reforms of the Roman Army and state, that would have a significant impact on the course of history.