This. By the time the Battle of France happens, the US military would still be stuck trying organize it's mobilizing formations. It wouldn't be in any condition to deploy overseas until '41, at the earliest. ....
I've looked at this from several angles. It is possible to bring three or four of the old Regular Army divisions up to speed in six to nine months. The down side is that ties up the cadre needed to mobilize at least three more new divisions, or bring the rest of the RA divisions up to speed within the year, or a smiliar number of National Guard divisions. So a short term gain is a long term loss. For political reasons I suspect the US would field one or two combat ready corps. As in the Great War a number of unready units would be sent to train in Europe, and take on French equipment.
Note the no other armies were well prepared in 1940. The Germans had in the line 20 divisions that had just formed up the previous winter & were armed with Cezch equipment. Those were barely half trained & even less capable than the French Series B divisions. Another dozen or so had been mobilized in the previous autum, had a mix of old and new equipment, and their training was incomplete. Only half the German aarmy had actual campaign or combat experience in Poland. One infantry division was stood down just before 10 May because of a flu epidemic among its horses. Its artillery and supply wagons were imobilized.
The French problems extended to their Series B units, which had mobilized in October, but spent the winter as construction labor & not training. The "Active" & Series A divisions were fairly well trained & performed adequately or well in actual combat.
The Brits fielded three corps of well trained soldiers, but the rest were a mass of half trained or untrained and unevenly equiped units. there were some strong similarities between the British Army of May 1940 & the US Army of November 1941 in terms of ready units.
The readiness of the US Army Ground forces was dragged out by several factors. One was a initial lack of urgency in the 1940 mobilization. Second were the downward revisions in in the size which caused changes (=delays) in the number of ground formations to be mobilized. Third was that twice the Army was reorganized to accomodate the change in division or unit structure. The square divisions mobilized in 1940 were over the next ten months convert to the triangular structure. In early 1942 a second reorg was started, based on the experience of 1940-41 training exercises. this went beyond the division structure & made substantial changes down to company, through the combat support, and to the service/support units. The change from rigid support regiments to flexible support groups occured at this time.
Bottom line there is, based on a September 1939 start, the US can field a combat ready field army by May 1940, but the larger it is the more units are delayed in readiness twelve to twentyfour months out.
Moving the mobilization ahead a few months can allow the National Guard units to reach a ready state. Since those came into Federal service partially trained it took ten to fourteen months to work those up. The variable seems to have been the competency of the division commander during the critical early months. Omar Bradley made his rep by taking over a 'failed' NG division & bringing it to combat readiness in a few months. Typically it took 18 months for the US Army to bring a new division up to speed from initial authorization, through activation, and to ready status. So, if the US mobilization is triggered by the Cezch crisis & the NG divisions come into active service in the spring or early summer of 1939 a half dozen of the eighteen available can be in France & usefull by May 1940.