He's not "investing in warfare", although any military spending could arguably be inferred as such. He's investing in the kind of warfare he expects the US to engage in should we (and the world) follow his envisioned nationalist path.

Conventional war could happen in all sorts of situations. The Bay of Pigs comes to mind as a pretty scary example. In this case we're talking primarily about Eastern nations, specifically China and Iran. The only thing preventing war with the east is Trumps friendliness with Russia, although that could dry up and leave us very vulnerable.

I could argue that since the precedent of the War on Terror has been set, Western armies have been tooled for asymmetric warfare for the eventual purpose of suppressing rebellion (policing). If we come under global governance why would a traditional war break out? It wouldn't, but you'd have plenty of dissent to deal with.

I'm really not sure if Trump is right in reverting to nationalism in the long term, but until something else is figured out that doesn't result in the raping of the planet or economic slavery I think it's a good idea to put the breaks on globalism for awhile.