Which factor was the greatest advantage of the United States in World War II?
It was her economic strength and physical isolation from the areas of conflict.
As soon as the Japanese failed to deliver a knock-out blow at Pearl Harbour, it was almost inevitable that the USA would win the war. America's industrial and economic strength was such that she could switch to a war economy and produce materials at a rate with which no country could compete.
This was helped by the fact that she did not suffer direct attack on the US mainland and thus her economy and industrial output was uninterrupted.