The US did not ratify the Versailles Treaty in real life; it made separate peace deals with the former CP powers in 1921, IIRC.
Largely because treaty ratification, including an alliance with France and/or Britain required a 2/3rds majority which did not exist in Congress at the time. Also there was a solid block of Senators who hated the treaty and would oppose it to the death:
en.wikipedia.org
They weren't enough to stop it, but the Republicans wanted reservations added and enough Democrats were fiercely opposed to any amendment to the treaty. Too many irreconcilable differences prevented any passing vote. Wilson was also a problem, as he refused to sign on even with a 2/3rds majority unless they accepted the treaty as is without reservations.
For a defensive alliance that was a non-starter, since too many senators opposed a treaty that took away the power of Congress to vote for war, as such a treaty would have required. Among the public the Irish and German population, IIRC a majority when combined, fiercely opposed the treaty, so would create major problems if it somehow passed.
And Yes, there is a possibility that public outrage could kill the idea of a peacetime Franco-Anglo-American alliance right after the end of World War I. AFAIK, the American public was less isolationist then than it would later become, but who knows, maybe it would have already been enough to put successful pressure on the US Congress to block this treaty? All they needed to do was to convince 1/3 of the US Congress in regards to this, after all. As for US military spending, maybe the logic would be that the US could quickly increase it if necessary just like it previously did in WWI and would subsequently do in WWII in real life.
The Irish-German block would block the alliance. They represented a majority of the public at the time. Once Germany quit the US public had zero interest in another European war.
I think the logic was that the US military was supposed to be defensive in nature, that is of the US proper. The USN was big enough to prevent an invasion as it was, so why have a large military for foreign adventures? I think the public looked at a weak army as a means of preventing the government from being able to get the US into wars at the drop of a hat. A long build up would be necessary before anything happened, which was in line with the public being willing to access if there was actually a national interest in fighting on foreign soil.
Do you really think that Poland would have accepted Hitler's August 1939 ultimatum if it wasn't for FDR? Or that Chamberlain would have avoided making a Polish guarantee in early 1939 had it not been for FDR? I do agree with you, though, that it would have obviously been better had WWII not occurred or, alternatively, had France not fallen in 1940 (except for young French gentile males of military age, of course).
Its a convoluted situation. FDR was pushing Chamberlain to take a more aggressive stance or get no help from the US if any war did break out in Europe and there are allegations that secretly US diplomats were telling the Poles to fight Germany instead of compromising and they could expect support for big territorial gains from Germany as well as military and economic support for the war. Due to the British guarantee it pretty much guaranteed that the Polish government would fighting over compromise, because it guaranteed Allied military support and if they didn't have to compromise they weren't going to. Plus they had a naive belief that Stalin would honor the non-aggression pact of 1932. The way 1939 played out it seemed that the Polish government had zero reason to negotiate and if anything a war would only solidify their hold on power, reduce the unemployed population, and net them very valuable economic territory from Germany in the peace treaty and weaken the threat from the west permanently as well as enhance Polish prestige enough to make their goal of being the 'middle power' of east-central Europe possible:
en.wikipedia.org
The March 1939 guarantee to Poland was in large part due to placating FDR. There was also the domestic situation to worry about, but I think that would have been possible without the March guarantee. That particular guarantee effectively ensured Poland wouldn't negotiate, since they had confidence that the Allies would back them.
