Thank You Marion. I guess they were correct in the film as far as Montgomery being in charge of all land forces on D-Day. For some darn reason I had it in my head that American forces were never under British jurisdiction during WWII.
Still learning,
Jim
Montgomery also commanded US Forces during the Battle of the Bulge. Because the near split in the American lines as a result of the "Bulge" created a difficult situation for command, Montgomery was given control of 2 of the 3 Armies from Omar Bradley's 12th Army Group. Since Bradley's HQ in the south had been isolated from the the 1st and 9th Armies in the North, they were attached to Monty's 21st Army Group. After the "Bulge" had been erased, the US 1st Army returned to Bradley but control of the US 9th Army remained with Montgomery through April 1945. As you can imagine Omar Bradley wasn't entirely happy with all of this, but it was Ike's call...
American ground forces in the CBI Theater in Southeast Asia were also under the command of British General Bill Slim until late 1944. In North Africa, the US 34th Infantry divison served under the British IX Corps for a while as well.
I would imagine British control of American forces on many levels occurred much more than one would think and vice versa...
Also believe it not, or at times US Army ground forces were even put under French command! 
In one particular instance, the US 36th Infantry Divison was put under the command of the French First Army during the battle of the Colmar Pocket.