Complex sentence: The total population of Roman Italy was estimated to be around 4 million before de second punic war, after the war the population was reduced to only 3.5 million and about 750,000 people died on the war.
Split into simpler sentences: The total population of Roman Italy was estimated to be around 4 million before the Second Punic War. After the war the population was reduced to only 3.5 million; about 750,000 people died in the war.