We don't fight wars by manufacturing large numbers of ships and planes in industrial plants manned by millions of workers. It used to be a Liberty Ship was launched every 24 hours, once the shipyards got cranked up, and thousands of B-17s rolled off the assembly lines every year.

Modern wars are fought with highly complex and meticulous machines and systems that take years to develop and assemble, spend most of their time being maintained and that are put together by relatively tiny numbers of highly trained and very well paid professionals. The money spent on weapons doesn't go back into the economy as wages. It goes into corporate profits or investment of more highly automated production capacity. There has been a "war dividend", a huge one, in fact, but it didn't wind up in the paychecks of the middle class. It went into the tax shelters and stock plans of their bosses.

Compared to their WWII predecessors, today's fighters and destroyers are highly complex machines that require years to go from specification to deployment and they cost enormous amounts of money, even after allowing for inflation. In a real shooting war against a similarly equipped and capable enemy, neither of us could build weapons fast enough to replace combat losses, or train the crews needed to operate them. Both sides would go to war with what they had in their stockpiles, and it would be all over in a hurry, as soon as one side ran out of some key system or component.

As for the "business cycle", its not clear how it would adapt to real modern war. Right now it doesn't seem to be operating particularly well in peacetime. American industry isn't geared to putting large numbers of people to work in a hurry any more. In fact, its organizing itself as much as it can to do just the opposite.