Complex sentence: World War II also saw the United States become by far the largest Naval power in the world, with over 70% of the world's total numbers and total tonnage of naval vessels of 1,000 tons or greater.
Split into simpler sentences: World War II also saw the United States become by far the largest Naval power in the world. In the late 20th and 21 centuries, the United States Navy possessed over 70% of the world's total numbers and total tonnage of naval vessels of 1,000 tons or greater.