[tanksoldier]

Something not widely recognized is that in 1938 the National Guard effectively ceased to exist in the US. All national guard divisons were federalized and integrated into the regular US Army in 1938. This increased available military manpower enormously and ensured those personnel were equipped and trained to a consistent standard by 1941.

Had this been delayed until actual hostilities it would have reduced the ability of the US to defend itself by a fair margin for a few years, and had Britain capitualated but the Commonwealth nations fought on would have severely limited the ability of the US to assist them while defending itself on two coasts.

This, combined with Britain's fall would have severely limited or eliminated the ability of the US to send aid to Russia as well, relieving Germany of an enormous burden on that front.

If Mexico had become actively hostile the stituation might have become almost untenable and possibly no aid would have been available.