*He had to keep all of the troops a secret.
*He had to throw the Germans off on where the invasion was going to be.
*He had to come up with supplies (Red Ball Express).
*He had to get all the armour/artillery, men, and had to wait until God gave him a break in the weather.
*He had to keep Montgomery and Patton in check.
*He had to figure out how to unload the boats (Mulberry
Harbors).
**These are just a few of the major obstacles he faced.
There are too many obstacles to list all.

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Only one.Invasion had to be in June because of the high tides and the weather had to be calm enough for landing craft..
Everything else had been taken care of months in advance by the joint chiefs of staff.General Montgomery was the Land commander, until September when the Germans were in full retreat. Ike took over the role as Land commander,together with his existing role of Supreme commander.

The Land commander was a political move by the USA, as it was election year for the President it was thought the American public would not accept a Limey in charge of American armies,irrespective that the Limey General had beaten the Germans in two major battles, where as Eisenhower was still waiting on a successful encounter with the Hun.