By chance a Japanese submarine is operating in the same area that the USS
Hornet is, rather than the picket ships of
OTL. She manages to fire a spread of torpedoes, which sink
Hornet with quite a bit of loss of life, including Doolittle, who's last seen trying to help others to safety. The sub itself is sunk by escorts, but the raid is obviously cancelled. The whole affair is an embarrassing debacle for the US.
The lack of
Hornet is even more keenly felt at the later disastrous Battle of Midway. Admiral Nimitz has been criticised by historians for insisting on making use of Japanese codes to attempt to destroy their fleet, but had he succeeded, American morale would have received a big boost. However, it was not to be, and the destruction of the USN's major units directly led to America's subsequent Japan-First strategy, against the heated arguments of Britain's premier, Winston Churchill.
The steady build up of American carriers and support forces led to the highly successful campaigns 1943-5, which saw Japan steadily forced back to the Home Islands. Eventually, the deployment of nuclear weapons brought about Japan's surrender.
In Europe, meanwhile, the lack of American support led to difficulties for the Red Army and also to post-war bitterness against America. The success of Operation Torch (the Normandy landings) of 1945, managed to secure France and the low Countries for the Western Allies, while Germany and most points east became little more than Soviet satellites.
That the sort of thing you're after?
