Answer: Oil and steel.
The US was by far the biggest producer and consumer of oil before, during and after World War II. For example, in 1940, its production was of 182.657 Mt (compare to the second place, the USSR, which was 29.700 Mt).
Additionally, the iron and steel production in the US was the only one that was not harmed during the war. In fact, the industry reached its maximum levels during and just after World War II. In 1945, the US produced 72% of the world's steel. By comparison, in 2014, the percentage was closer to 5.3%.
