Militaristic warfare is old-fashioned and quickly leads to instability tbh. It would be better for Thanos to focus on economic warfare, since that comes with all the benefits of military warfare, removes a lot of the negatives of MW and is also more sustainable and is, therefore, just a better strategy... I think?
But, yes. Thanos could flatten the U.S. with his army. And then he would flatten the rest of the world and no one would understand his motives other than him being a warmonger.