When there is no market, you can't expect it to produce anything. The usual capitalistic incentives only work when there is time for the market to react and re-organise. During war time there is no such time. If you let the free market dictate everything, the capitalists would've sided with Hitler because it would be more profitable and less costly to side with the more likely victor (and Hitler would've won if the rest of the developed world hadn't banded together to strike back). The US was already supplying Nazi Germany with steel and trucks and had absolutely no trouble in doing that because there was more money to be made in this war machine.