Let's focus on the Pacific War. How does this change the course of the war?
Well, I have some ideas.
Firstly, discount any and all thoughts of an early ground campaign, so no US led invasions of defended Japanese held islands before mid 1942.
Secondly, move the few existing ground forces from rear areas and vital strategic strongholds (I'm looking at you, Oahu and at you, PCZ) to hastily established forward bases.
Thirdly, while digging in on new islands, build airbases and relocate the airforces as well.
Midway and Wake would each get some love, and that is easy, as these historically were garrisoned by the USA.
Guadalcanal. Send in the US ground forces in July, 1941 and build an airstrip there,
Do the same for the best island in the Gilberts, no free expansion for the Japanese!
Take all the rest of these relocated forces and establish them in New Britain and New Guinea, along with the bulk of the airforces.
This is a fair attempt at showing what this part might look like...
So, the USA risks everything, on meeting the Japanese before they can expand to the SE, securing Australia and NZ, and forcing the Japanese to fight for these islands that historically they got a toe hold on with little trouble in OTL.
So, say on June 25th, 1941, something like the above is decided upon, and the US forces are busy implementing this by Sep 7th, 1941.
In addition to the US redeployments of ground and airforces...
Send all US pacific and atlantic fleet submarines to the Philippine islands, and invite the British and Dutch (And any Aussie/Kiwi, as well) to also redeploy all their submarines to the PI, as well. Confine the offensive to the naval forces combined submarine offensive, and airforces in the PI are also given an unlimited hunting license for attacks on any Japanese forces/shipping they come across.
I'm too tired to dig up British and Dutch markers, but look at the PI's as a combined allied sub and airforce base.
Getting tired, but here is an attempt to show allied bases to prevent Japanese easy flanking attacks.