What is the strongest fictional military that a US/UK alliance with prep could defeat in a all out war scenario?

US and UK are chosen as earths champions and pretty much every country is throwing their GDP at both of them, they are made aware by some intergalatic eldrich diety they will fight side by side once more but they weren't given the 5 W's (who what when where or why). Both of military industrial complexes are supercharged and production goes into overtime.

Both are given 10 years of paranoid spine chilling fear and paranoia level prep to boost their militaries and defenses. What military could they defeat together, yaujta? Imperium of man? The flood? Thanos MCU army? Chituari? Chimera? Solo the devastation of baal?

Keep in mind this is an all out war not just 1 pitched battle. Both the US and UK are being funded by the rest of earth meaning production and logistics also comes from everyone else.