> It doesn't go up in smoke, it goes back into the economy.

There is still a gigantic opportunity cost of bringing together resources to produce an explosion rather then something more useful.

The old 'its going back into the economy' is some sneaky Keynsian language that only applies in certain macro economic situation and even that was mostly wrong and a misunderstanding of economic theory in the late 30s.

A large proportion of it is just destroyed. Along with human lives. Sure the money seems to come back around, but that's an accounting trick: it's printed by the guys paying anyway. The human labour and lives invested are gone, unless what they did is one of the fraction of the work permitted to spill into civilian life. A million man hours go into a warship's hull alone. What else could those hands and minds have done?

You can fund medical research without spending a million dollars sending a man to a desert, having his legs blown off, and flying him home. Hell, the US especially could actually use that research on civilians without bankrupting them!

And what about rebuilding bombed out cities? Is there so little else to improve?

> the money seems to come back around, but that's an accounting trick

It’s also a sociological trick. Societies support massive military R&D in ways that are impossible for public projects. We could wish upon a star that humans weren’t as we are. Or we can work with reality.

>> The human labour and lives invested are gone

Military innovation is also historically a jobs program.

Check out how many states & companies are part of R&D/Manufacturing for the F-35.