This is demonstrably false. You need only look at the actions of our government (e.g. The CIA) to discern that the United States has gone to war (or "military conflicts" or whatever they call them these days) for reasons such as A) financial gain for private persons and companies B) financial gain for politicians C) financial gain for the country D) political gain for individuals E) extending a miltary based hegemony F) moral agendas (e.g the War on Drugs) etc, etc. The United States is by no means some perfect rational actor which engages in war solely for economic reasons or for "self-defense".