Letting all of your potential allies be destroyed before you lift a finger is a bad strategy if you actually want to have allies for the fight. If the US wouldn't have gone to war while Britain was still in it things would have been very dark indeed. In that case it would be unlikely that the US would liberate Europe itself from the continental US. Europe's future would be slavery under the Soviet Union, Germany, or Italy. That wouldn't bode well for the US.
As directed by Moscow, Communists in the US and Europe opposed war to to aid that other "fine bunch of socialists" running Germany. That changed once the Soviet Union was invaded.
Using the power of the state to commit mass slaughter of civilians whether by violence or starvation is a very different thing than open warfare against a peer state.