I feel I must elaborate on my previous post. From the perspective of Marshall, an invasion of Europe in late 1942 could have only two possible outcomes.
1. The invasion gains a foothold, and the war is shortened by as much as 30 months.
2. The invasion force is thrown back into the sea. A maximum of five American divisions are destroyed. The USSR makes use of the draw of German forces to the West to rebuild. The five American divisions are rebuilt in a matter of months, and the worst case scenario is that both sides end up right back where they started.
If Marshall really thought as you claim he should have been shot as a German collaborator!
Five Divisions would be more than 100.000 men and a large part of the trained forces available in 1942 - throwing them away in a failure can/will lead to unpredicatable political reactions. Both the US and British armies actually had problems manning their combat Divisions, especially with capable infantrymen.
If 5 Divisions is all that the allies are willing to risk, the North African and Italian options from OTL would provide much better bang for the buck, incl. a much lower risk.
Try and read Rick Atkinson's book on the North African campaign (Army at Dawn) and you will see how far the US Army was from being an efficient combat machine in 1942.
Anyway the number of Divisions availabale is only one factor, at least as important was landing craft and logistic capacity. in 1942 the allies were hard pressed in the North Atlantic and shipping tonnage was in acute shortage. So even if a large port could have been taken early in an invasion, the allies would seriously have risked not having the ships to unload.
A basic allied strategic problem was that the Germans, being on the shorter inner circle and with a very efficient railway system (even under allied airspremacy) much faster could move troops from one front to another. In short the Germans, certainly by 1942, could concentrate forces at any landing place in France much faster than the allies could.
If Churchill, as already said by other, had preserving the Empire as his first priority, he would have taken up the German approaches in 1940-41 - they basically promised the British to keep their Empire if they left the Germans alone in Europe. Actually I think you can say that the British sacrificed their Empire in order to save the world from the nazis. The Americans OTOH got world supremacy handed over almost for free, as the previous "global policemen" had become too exhausted (from fighting the nazis) to stay in the job.
That doesn't change that the Americans IMO have proved to be the best "Golbal cops" ever seen in history, as it doesn't change that they got the job under very favourable conditions.
Regards
Steffen Redbeard