He was well protected on the north, where his troops occupied Norway and Denmark. To the south, the immense barrier of the Alps stood between Germany and the Allied forces, who in any case were still south of Rome. Hitler was not even excessively worried about his eastern flank: his armies were 600 miles east of Warsaw, and within 300 miles of Moscow. He had lost the Ukraine in 1943, much his biggest loss to date, but for compensation he had held on in the Balkans and was still besieging Leningrad. On all fronts except one he had a deep buffer between himself and his enemies. That one exception was to the west.

The Allied forces building up in the United Kingdom, now 2,500,000 strong, were the greatest threat to Cologne and Germany's industrial heartland. Not only were they much closer than the Red Army, they were operating from a virtually impregnable base and had far greater mobility than either the German or Russian armies. But of course there was the English Channel between Hitler's Europe and the armies gathering in the United Kingdom. Hitler knew, from intensive study of the plans for operation Sea Lion, a German invasion of Britain in 1940, just how difficult a cross-Channel attack would be.

Hitler did what he could to make it even more difficult. Just as the British started thinking about returning to the Continent even as they were leaving Dunkirk, so did Hitler begin thinking then of how to repulse an invasion. First the ports were fortified, protected by big guns on the cliffs, by machine-gun emplacements, by trenches, by mine fields and barbed wire, by underwater obstacles, by every device known to German engineers. The Canadians learned how effective these were at Dieppe in August, 1942, when they were met by a veritable wall of steel hurtling down on them from every direction. In 1943, the Germans began extending the fortifications up and down the coast; in January, 1944, with Rommel's arrival to take command of Army Group B, construction reached an almost frenzied pace. The Germans knew that the second front had to come that spring, and that throwing the invaders back was their single best chance to win the war.