PDR, the US did not have to join in the war against Germany in the same way that Russia did not join the war against Japan.
As a neutral the USA did not supply war material directly to the UK. Had they remain as a friendly neutral they could have built Liberty ships but not tanks. The air war against Fortress Europe would have been a night war with Germany still committed to maintaining day fighters lest we switched tactics.
Africa could have dragged on much longer but logistics would have remained the Achilles heel for Rommel. D-Day would not have happened in 1944.
The atom bomb would not have been developed when it was.
The US would have been down to one aircraft carrier in 1943.
They would not have developed H2X when they did. Packard would not have built Merlins.
Russia would not have had the convoys.
Stalin would have been defeated.
Germany would control Europe.