D-Day called for one of the most critical spread and focus strategies
in history. The Allies needed an entry into Europe that would maximize
their chances of success while minimizing the risk to lives and
military assets. The map of Europe prior to D-Day illustrates how they
arrived at their brilliant solution.
First, look at the extreme
breadth of the Russian front. Keeping Germany occupied with a
two-front war, which meant maintaining a coalition with Stalin and the
Soviets, was a critical element of Allied strategy. As Americans, we
remember the drama of D-Day, the liberation of Paris, and the Battle of
the Bulge. But it was the “Barbarossa” campaign—a battle of
unprecedented size between Russia and Germany—that made the invasion
from the West possible. More than a million German soldiers were
occupied on the eastern front on D-Day, when the Allies invaded the
western front.
Second, take a glance at Scotland, facing Norway
to the east. Norway was important to Hitler as the home of his U-boat
bases. Taking advantage of the geographic proximity of Scotland to
northern Europe, Allied leaders developed a strategy called Fortitude
North. The Fortitude North ruse was intended to freeze in place the
thirteen army divisions (over 130,000 soldiers, not to mention 90,000
navy and 60,000 Luftwaffe personnel) that Hitler had stationed in
Norway, Denmark, and Finland. [note – I have seen estimates up to 27
divisions.] [1] To create a credible threat to the north, the Allies
needed some loud saber-rattling in Scotland.
http://www.strategybydesign.org/d-day-strategy-and-the-normandy-invasion/