Consider the League of Nations with the US included
Looking at things, (And as much as I'd LOVE an Irish-American union for a TL) I'm beginning to think that American territory in Europe is very difficult to near impossible.
And that was my first reaction. I didn't and still don't know
why you wanted to specify this particular means of entangling the USA into the war earlier; more plausible ways of getting that result come quickly to mind.
(And unless someone brings in a new angle, I've shot my bolt on Ireland here and won't, um, harp on it any more!)
A way of getting some kind of territorial involvement for WWII would be if the USA had gone ahead and joined the League of Nations after WWI. Which is part of why this was opposed--but actually, according to Richard J. Barnet in
The Rockets' Red Glare, many of Wilson's opponents, including Lodge, were
also on record long before the war as favoring some kind of international peacekeeping body, and had Wilson been able to accept some sort of compromise he probably could have gotten a majority for a revised treaty in the Senate. If we imagine this happened, or that Wilson got shunted aside, then, if the revised version was in turn acceptable to the Versailles powers, the USA would be a charter member of the League, and probably also a signatory of the Versailles Treaty as well. (OTL, the USA never did sign Versailles and we eventually made a separate peace with Germany and the other Central Powers).
Thus, as a League member, all sorts of possibilities would have been open for the USA to be granted Mandate authority--perhaps over places like both Heligoland and Danzig! I can see the League choosing America as the Mandate power for any territory carved out of a major European state, precisely because the USA did
not have other territorial interests there, whereas giving it either to Britain or France would have been tantamount to old-fashioned conquest, despite the restrictions League mandates theoretically imposed.
All I know about Heligoland is what it says on the Wikipedia page, but from the maps there it looks, as I said somewhere above, hella tiny.
Now on the other hand, besides Danzig, there was also--the Rhineland. Which I believe US forces actually did occupy a portion of immediately after the Armistice. I can see the League handing over the responsibility of keeping the Rhineland demilitarized to the USA, under long-term Mandate authority, rather than Britain and France trying to hold it. And, given the conciliatory mood of American policy toward Germany during Weimar, perhaps the Germans would tolerate a minimal US presence there right up to Hitler's seizure of power in 1933. Then, when Hitler wanted to march into the Rhineland, it would be against at least a token US force there.
Since 1935 is way
too early for Hitler to start the war, I suppose that the Americans would yield, at least if the League ordered them out. But if the USA held some other mandate near or partitioned from Germany, there would be your tripwire, with Americans already once burned and very wary and thus perhaps hairtriggered to join the Allies immediately upon the invasion of Poland. Indeed the League might continue, despite having failed as spectacularly as it actually did OTL, with one of its founding powers based safely across the Atlantic, and the Allies would operate as the League fighting renegade powers. It would be the League rather than the UN that might eventually win the war.
I did once see a map, proposed early in World War II by the British, to foist Palestine off on the USA to gdt it away from the Ottomans, yet not be saddled with the responsibility for it.
See, now you are talking about yet another League mandate, OTL, if your reference to "World War II" above is a typo for WWI as I assume. Because Britain did hold Palestine as a League mandate, just as France held Syria/Lebanon.
Not to suggest the USA would have been handed every single League Mandate in the whole Europe/Mediterranean region! Mix and match as you like.