Some Americans have been on this train for over two decades and seen this ebb and flow of warmongering more than a few times.
When it comes to war Americans won't oppose it if it doesn't impact them personally. If the war means gas prices shoot up, they'll oppose it. If prices come down they'll support it.
Also many Americans also rely on the military industrial complex to make a living. They may not support a given war, but their livelihood depends on wars being fought around the world.