The threat embodied by the Soviet Union under Joseph Stalin dampened any comeback of isolationism.
The ideological goals of the fascist powers in Europe during World War II and the growing aggression of Germany led many Americans to fear for the security of their nation, and thus call for an end to the US policy of isolationism.
The Emancipation Proclamation freed slaves living in ten states that were still in rebellion. Slaves that were already in a state that was taken over before the EP were not freed. At the same time, all those freed slaves were enlisted in the Union Army. As a result, after some time black slaves and white citizens of the USA fought together against one enemy. No doubt, the Civil War united black and white people as well as reduced tensions existing among them. The majority of freed black slaves fought rather valiantly and earned the respect of whites. This promoted equality and eradicated some racial prejudices in the society.
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Operation Barbarossa was the Nazi German military invasion of the "USSR", since this was the famous surprise attack by Germany that broke the Nazi-Soviet Pact in World War II.
