Nickel was used in the alloy for full metal jacketed bullets, which were expended by the billions, also stainless steel was needed in unprecedented amounts for everything from canteen cups, to skin metal for aircraft, and nickel is the metal that alloys with steel to produce stainless. Most people today are unaware of what a titantic struggle that war was, a Sherman tank took 67,000 pounds, 33 tons, of steel to produce one tank, and we built enough of these that if lined up bumper to bumper they would make a solid line from Chicago to New York, that's a lot of steel. How many aircraft, ships, artillery pieces, small arms, helmets by the millions, jeeps, half tracks, knives, bayonets, buttons, belt buckles, ball bearings, bombs, torpedoes, and artillery shells by the millions. They needed the nickel for the war and decided to make the 5c coin from silver, which they figured they could retrieve after the war was over, because they needed all the steel they could get for the war.
We lost over 420,000 men killed in 3 3/4 years as compared to Viet-Nam with 52,000 men KIA in 10 years. Nam was Hell, and WWII was HELL ON WHEELS!!