I'm not so sure about that. Germany's demand for both the Rhineland and the Sudetenland was viewed as legitimate by many people in the West.
I don't see American boys being willing to die for either of these two territories.
The French & Belgians had been. Both nations invaded Germany in 1923 to enforce the Versailles Treaty. Lack of support from their former allies, Britain, Italy, and most important the US pulled the rug out from under France. President Coolidge actually went with the pro German factions in the US undercutting its ally France, effectively eliminating the remnants of the Entente. With continued solid support from the Entente nations France would have retained a proactive and aggressive policy keeping Germany down. Left without any major allies France and Belgium reversed their policy, after another four years of debate. Finally in 1928 the Chamber of Deputies endorsed a policy reversal to the defense by voting for a fortress construction project, vs funding a modernized offensive capable army.
A engaged and anti German US means the French government can act with confidence against German resurgence.
Thats the stick. There could be a carrot with Entente cooperation in working out a better solution than the Versailles Treaty.
If you are talking about the US coming in late 1939 say October. There is no way that US troops can get to Europe in the numbers needed to stop the Germans against France. The US is just gearing up and only has three understrength infantry divisions. The national guard is being called up but again the twenty or so divisions are in worse shape and will need plenty of training. In OTL the draft was only reintroduced in 1940 after the fall of France. Since the US is in the war, I assume that the draft law will be passed right after the declaration. The problem is there is nowhere to put the draftees (the US had to construct a large number camps to house them). If they try and send the 1st-3rd divisions, there is no one to train the draftees. The only way I see France surviving is the generals remove Hitler and then sue for a white peace which is accepted.
All this assumes a US engaged in European matters shapes the same military as it did for a isolationist policy. Which does not make sense. The Army & National Guard that existed in 1938 were shaped to a large extent by War Plan Orange. That plan contemplated mobilizing less than 500,000 Army/NG, and fielding a expeditionary force of only 100,000 twelve months after initial mobilization. Active engagement in Europe means Congress would have to take more seriously funding a larger mobilization plan that aimed at standing up between 1.5 & 2 million soldiers in twelve months. As it was the US came close to having 1.6 million organized and trained men for the Army by the end of 1941. However the mid 1941 decision to expand from 45 odd ground combat divisions to over 150, and increase the Air Force five fold shrank the possible expeditionary force of December 1941 from 20+ divisions to maybe a half dozen.
The butterflies of a European engagement for the US extend far beyond military alliances. Anything like the Smoot-Hawley Act is very unlikely. With no catastrophic tariff war during the Depression years the economic retrenchment & reforms will be much less painful and lengthly. There likely to be more inter government cooperation to alleviate problems in the banks & other fixes. That makes it harder for the nazis to gain voting traction 1930-33. & their bogeyman the communists. With a shallower recession the NSDAP is going to peak out among voters sooner over economic matters.