With a J1DOW and Japan taking Calcutta meaning to swing back and take Hawaii as well ~J8, the allies can follow 1 of 2 basic strategies in Europe:
USA invests heavily in Europe, opening a second front in Germany’s backyard. This forces the UK to go heavy into the ME with 2 ICs in Egypt/Iraq/Persia (3 ICs is perhaps no unnecessary luxury in this scenario). This is because Calcutta needs to be taken back once Japan grabs Hawaii. A little bit from what comes out of those ICs can maybe get into Moscow as well to aid Russia in keeping it. The UK will have only minor bits of IPCs left to assist the USA in Western Europe (~10/turn, TRS as best buy).
USA invests only mildly in Europe, opening a second front in Germany’s backyard with massive UK aid (not such heavy UK presence in the ME as above). Typically USA serves only as a can-opening threat for UK-follow up. Example: USA takes Norway + Denmark, UK takes Berlin (If Germany ignores the allies and keeps throwing everything at Russia).
Whatever strategy is followed, the Wallies must somehow make sure their threat is in place G5, so that Germany will loose either WG, Rome or Berlin if it builds all aircraft for a G6 assault on Moscow (depending how many investments the USA made and thus, how many the UK was able to assist).
Typical forces USA can expect in Europe with heavy investments: 2DD, 2CV, 5FTR, 1TAC, ~4STR and 2TRS per turn (fully loaded). Early TRS can ofc be filled with Brazilians and other starting units. Once Normany and SF are taken (and/or WG), USA also produces from those factories. US6 should see 27US and 8UK land units and 9 USAF in or approaching Western Europe. A bit too much for Germany/Italy if they focused everything on Russia all the way into G5 and only start building troops against the wallies G6 (10 in Berling and WG each can be overrun -one of them if not both), so if this doesn’t arrest German flow of reinforcements into Russia I don’t know what does…
Investing ‘mildly’ still sees the USA with 2DD, 2CV, 3FTR, 1TAC, 3STR, 6TRS fully loaded.
Because the USA should be able to keep Japan out of Hawaii/Sydney with the extra IPCs put into the Pacific, the UK can put more effort in Western Europe at the cost of the ME, starting a war of attrition with Germany. The few troops the USA has should not be ‘kamikazed’ but used as a can-opening threat (Berlin, see above) and to capture areas Germany cannot recapture (Norway, for example).
On a sidenote: a couple of RAF-FTR really enhances Russian defenses in Moscow. The more ships/early bombers Germany builds, the less RAF the UK needs to send. With Max German land production (i.e. no ships/early bombers at all), Moscow may need 10-12 Spitfires/Mosquito’s to withstand the assault.
These are only the basics of what I know the allies can do. They must be adjusted whenever the Axis adjust… If, for example, the USA invests heavily into Europe and Japan does not look likely to swing back at Sydney/Hawaii but rather continues to invest heavily in Mainland Asia/Russia, the UK may ease their spending in the ME a bit and assist the USA a while in spending much more for western Europe for a couple of turns…