At the start of the Korean War, the United States found itself low on the supplies it needed to help South Korea beat back an invasion by North Korea. But this was 1950 — and the heavily industrialized United States quickly ramped up production and helped push back the invasion, leading to the Armistice that remains in place today.
That is an oversimplified telling of a very complicated conflict, of course, but the point is that the United States had a robust defense industrial base in place that allowed it move to a war footing relatively quickly.
