War is merely politics by other means. It is the last resort to achieve your aims. We still see people make use of it - when they don't get what they want by standard means, they riot, for instance. This is merely done on a larger scale. So 'obsolete' is a silly way of looking at it. It depends what you want.
Britain spent many years suppressing the Slave Trade, for moral reasons, with no economic advantage for themselves. Was this wasted effort? Are there not times when war is required to safeguard beliefs, values and safety?
Wendell Wilky was defeated by FDR in the election just prior to US entry to WWII. He was more of an isolationist on economic grounds. If he was President, then WWII would have gone on longer, perhaps ending in Axis victory or a negotiated armistice. Without Lend Lease to Britain, she might not have been able to stand alone as effectively against Germany. This same economic argument would have left the world far worse off, I think. Sometimes the bad guys need to be clobbered with a stick, although you should walk softly, as Teddy Roosevelt said.
People often went to war for spoils or economic reasons in olden days, but often not. They went to war to defend their hearths, to defend their beliefs, to uphold their worldview, as well.
If economically war is 'obsolete' in the sense of spoil and pillage, perhaps, but the threat of war still needs to loom to keep players acting nice mostly. It is the Prisoner's dilemma of Game Theory in a sense, for you could run roughshod over foreigners, seizing property or companies say, and still gain economically otherwise. This hypothesis of the article is simplistic, almost Marxist, in its primacy given to macro-economic concerns of trade and so forth. I think its reasoning of the Napoleonic soldiery merely out for lucre laughable though, as that certainly was not applicable there.
How much bellicosity the US needs to project depends on whether you think the US has ideological or moral imperitives or rights to intervene in other societies. Some is needed, to keep rivals in line and Americans abroad respected, or keep trade routes open. Getting the balance right is tricky. Often a Power acts as policeman to keep free trade flowing, as Britain used to and the US does today. Looking back to isolationist days makes no sense if no one shoulders the burden of deterrence, and to pretend it isn't somewhat necessary is a facile dream. Countries don't act more level-headed than a bunch of drunks in a bar, or schoolchildren marooned on an island.
Sometimes War is required, or at least the threat thereof. As Vegetius said: Qui desiderat pacem praeparet bellum - if you want peace, you must be prepared for war.
