Yes, they do.

> they invest.

Even if all government spending was investment, investment is just spending in the expectation of future return.

> It's not like there were trillions of dollars sitting around waiting to be spent.

It is literally exactly like that.

> The real tragedy of war is not that trillions were spent

True, that's not the real tragedy of war.

But that's because spending money isn’t tragedy (though the opportunity cost of the choice of how to spend it can be), not because government spending is not-spending, so the trillions spent on war are not-spent.

> Your argument of efficiency is inherently flawed. It implies that war is good as long as it's cost effective, which isn't true.

I thought the context was clear that I was referring to utilitarian justification, whether it is good is a differebt story, sure.