Again, the answer to this question is neither simple nor straightforward. On the one hand, the Western Allies (the USA, Great Britain, France, Canada, Australia, etc.) fought to liberate Western Europe (directly) and Eastern Europe (indirectly) from brutal and murderous Nazi occupation.
And thus put an end to mass murders and other crimes being committed by the Nazis on almost daily basis (mostly in Eastern Europe). Which was, obviously, a very important service to God.
On the other hand, Western Allies were the allies of the Soviet Union (i.e., of Joseph Stalin) who fully intended to conquer the world, destroy the global human civilization and replace it with an alternative civilization – global Union of Soviet Socialist Republics. Which was, obviously, a very important service to the Devil (because ultimately it would have established a genuine Hell on Earth).
Still, by invading and occupying Western Europe (including Western Germany) with mighty military force, by manufacturing a gigantic fleet of strategic bombers (B-17s, B-24s, Lancasters and especially all-but invincible at the time B-29 plus highly efficient escort fighters – P-51s and P-47s) and, what was the most important, by developing and detonating the first atomic bomb (and later using it against the Japanese cities), the Allies created a very powerful deterrent for Stalin’s ambitions.
Which ultimately prevented the latter from achieving his Satanic (literally) objectives. And thus provided a vitally important services for the Christian God.
Consequently, the only correct answer to this fundamental question is this: actions of the Soviet Union (like those of many other people) served both God and Devil. But by preventing Stalin from destroying the Christian Church and the whole Christian civilization, the Allies served God far more than they served Devil.