I wonder if the explanation can't be found in the sophistication of the weaponry - - say that Viet Nam was being fought with the equivalent of WWII weaponry, with some technical improvements, but the later wars were with highly advanced space-age weapons. Look at the casualty rates, for example: in Nam, 200 deaths a week were relatively common casualty counts, if those same counts had appeared in Ireq, Bush and Cheney probably woulda bin lynched. Maybe they found a way of fighting wars with minimal casualties but at a hugely increased cost, including the cost of lifetime care for all of the casualties who in an earlier generation of warriors would have just died of their wounds but were saved by advances in battlefield medicine.
You also have to wonder at the cost of Star Wars, which supposedly through the genius of Ronald Reagan, bankrupted the U.S.S.R. but miraculously spared the U.S.A. even though both sides had to spend their brains out on the technology and deployment.
I hope that if the figures in the graph are analyzed more, military spending will be seen as the culprit, in which case, conservatives, who don't give a shit about human life and pain, but sure as hell love what's in their wallets, will finally find a reason to halt the deadly spread of militarism where it's easiest to stop it - - at the appropriations level.