The simplest explanation would be to have the attack on Pearl Harbor not be the overwhelming victory it was. Pearl Harbor is the classic example of a tactical triumph being a strategic disaster. It united a divided and isolationist America and filled everyone with a determination to fight to the end.
Now what if Pearl Harbor were still launched and still succeeded but was not nearly the disaster it was in our time line?
At 6:45 the US destroyer Ward sent the following message:
"We have attacked, fired upon, and dropped depth charges on a submarine operating in defensive sea areas."
They had run into one of the five mini-subs the Japanese had sent in ahead as part of their surprise attack. Now given the fact Admiral King had received a ‘war warning’ recently this report should have sent the base into alert. Instead it was assumed that the sub was imaginary, that they were chasing shadows. Behind this was the basic belief that the Japanese would never dare attack Pearl; the Philippines, Wake, or Guam perhaps but not Pearl.
What if the mood at naval headquarters were just a bit more paranoid or nervous? What if the warning were taken at face value?
The first planes will not launch from the Japanese carriers until 7:40. There is enough time, if the message is taken seriously, to sound the alert have a combat air patrol formed and much more importantly have the fleet leave port and make for open water making them much less vulnerable.
Nagumo had orders to carry out the attack even if his fleet were discovered. The planes still would have gone in but rather than finding aircraft lined up wing to wing and ships at berth US fighters are attacking them and the warships are moving targets filling the sky with anti-aircraft fire.
The Japanese still have surprise on their side and the Americans are confused while the P – 36’s, P – 40’s, and F4F’s are no match for the Zeroes. They do however do some damage to the Kates and Vals while making their attack runs much more difficult.
USS West Virginia is sunk while Arizona and Nevada are both hit and damaged. The US loses a total of 63 planes in the air and on the ground. The Japanese lose 47, mostly torpedo planes and dive bombers. There is damage done to Ford Island and to the various air bases and facilities; none of it will be deemed critical. Despite having an active radar station the situation is too confused to take advantage of it.
No US counter attacks are launched or even attempted.
Recovering his second strike Nagumo decides to immediately withdraw. He is disappointed at the scope of the victory, but it is victory. Though he understands that the goal of destroying the US Pacific Fleet has not been met.
Also let us assume that the Japanese ambassadors were not delayed and delivered their country’s declaration of war to Secretary Hull on time. The fury against the surprise attack is still great, just not quite as ferocious as it might have been.
With the Pacific Fleet still intact and all three front line carriers untouched the navy would have gone out to fight for the Philippines. Wake and Guam would also have been saved and then reinforced, badly damaging Japanese plans. With the navy intact the army in the Philippines could be supplied and reinforced; meaning no Corregidor and no Bataan.
The Japanese may still succeed in taking the Dutch East Indies as well as Malaysia and Singapore but they are in no position to threaten New Zealand or Australia, never mind India. If they are forced into a grinding battle against the US in the Philippines they are never able to create a defensive perimeter around the Home Islands. Japan itself is threatened with bombing from 1942 on and more importantly its shipping will be attacked from air and naval bases on Luzon and the other islands.
The irony is that all this might be a POSITIVE for Japan IF Hirohito is able to enforce his will on his military and convince them that a negotiated settlement is in their best interest. Also assuming this takes place prior to the Allied demand for unconditional surrender. Given the circumstances FDR might have been open to negotiations in order to fully concentrate on defeating Germany. They would have to yield up their gains in the Pacific, Indonesia, and (bitter pill) China. But just might be able to keep Manchuria and Korea. Most importantly they could avoid the utter defeat and conquest of the original time line if they were never in a strong enough position to think they could wear down the US.