A few months after Pearl Harbor when another U.S. ship gets sunk by a U-Boat it gets played up and FDR will have the votes for war.
But, he likely won't have the support in the country for a Germany first strategy without Germany declaring war first.
Agreed on time frame, FDR was too talent for it to go much over 4-5 months.
FDR still will do a Germany first, just not as strong a Germany first. It might be a 20/80 Japan/Germany not the "10/90" of OTL.
A Germany first strategy doesn't just make strategic sense, a Japan first strategy is very hard to pull off immediately. After 1943 sure but ignoring Germany won't necessarily lead to greater action against Japan.
It can partially be pulled off. The net effect at first will not be huge, but with war with Germany delayed until March 1942, these are the type of actions that might happen.
1) 1-5 more capital ships moved to Pacific, or moved earlier. Now, once the war starts with Germany, it will be reversed, so the effect is temporary. We might see some more Iowa class or Essex classes in the Atlantic, or as new ships become operation, we could see things like the Wasp being transferred back to the Atlantic.
2) Pearl Harbor was screaming for air replacement and reinforcements. The B-17 was supposed to have a naval role. We could see more squadrons of Heavy bombers enter the Pacific theater and never leave.
3) We might see a few more divisions in the Pacific, such as extra units in Hawaii or Australia, or even somewhere radical like Burma. Again, over time, it likely goes back to our force levels. I can see a situation where the USA is defending Australia but more Australian units are in Europe/Africa.
Until war starts with Germany, FDR will be hard pressed politically not to send more units than OTL to the Pacific, and it will take time for him to get back to the force deployment of OTL. For example, if the units assigned to Operation Torch are used in the Pacific, FDR may just wait until more units become operationally ready, instead of pulling a unit from Australia to attack Morocco. It will not be huge, but the timeline in Europe probably runs a few months (1-3) behind OTL until late 43 or 44.