For years leading up to that point Congress was passing "neutral" bills that allowed the country to sell weapons to the Allies but not the Axis. Since we're discussing weapons production, yes, the US joined the war effort on the side of the Allies well before formally declaring war.

Yes, there were isolationists, but there weren't enough to change the fact that fascist/communist victories in Europe would be bad for America. The neoliberal thing where we support liberal democracies is an evolution of the Monroe doctrine, because we've recognized for a long time that the best path forward for America is free trade and interdependency with other like-minded countries.