I would say brinkmanship and groupthink are a much more likely explanation than theories about defense spending or oil production or currencies, which fall flat on analysis. Not to say that people didn't make hay with government contracts, but there are easier ways to swindle public cash than to get high-placed politicians to start wars.

It also leaves out a very significant factor : vote winning. Patriotism wins elections, because it, in of itself, is just another form of groupthink.