Americans, like the citizens of any empire before them, are…

Americans, like the citizens of any empire before them, are only anti-war if the war isn't in their own interest.
If the war makes sure their standard of living goes up, or at least remains the same, it is acceptable to them.
But now things have changed significantly: The wars are a desastre for their standard of living. In fact, the wars seem to be designed to destroy the American standard of living.
That is the only reason there is any significant American anti-war movement right now.
The more obvious it becomes that these wars are waged not for Americans, but against Americans, by their own "government", the more the anti-war side will grow.
As for now, a lot of people still cling to the idea that someone in power is on their side and deserve their support, which is, of course, hilariously ridiculous.