THE FIGHT IN EUROPE
The first challenge the U.S. faced in fighting for Europe was getting supplies through the well-armed wolf packs of German submarines. In the first months of the war, the Germans sunk more than 500 American merchant ships — faster than the U.S. could rebuild them. Without supplies, Britain couldn't continue fighting.
Faced with a high-tech sub threat, the U.S. and Britain devised cutting-edge solutions. The British broke the German codes so the Allies had an idea of where the German submarine packs were hiding. Patrol planes and convoys equipped with sonar attacked the subs.
Over a year, the tables turned. The German subs had to draw back as the Allies sunk more than a hundred of their undersea boats, in some months at the rate of almost one a day. With fewer German subs, Allied supply ships were safer; their losses decreased from the equivalent of 75 Liberty ships sunk a month to fewer than 20. Although the Germans kept up some submarine attacks for the rest of the war, the time for subs to make a real difference had passed.
British and U.S. air attacks on Germany
The British believed that heavily bombing German cities would break the German will to fight (even though when the Germans bombed Britain, it only made the British tougher). In late 1942, Britain sent more than 1,000 bombers to attack the German city of Cologne.
Now that America was in the war, the Germans were under constant attack from the British at night and the Americans by day. Heavy conventional bombing didn't break the Germans' will to fight or even stop production; the Germans made planes and submarines until the end of the conflict. What bombing did was to open up a second front in the air before the Allies invaded Europe; until that point, in the first front against the Germans on the ground in Russia, only Soviet troops were fighting .
Allied strategic air attacks forced the Germans to spend limited resources on protecting civilians, using their guns and fighter planes. Domination of the air over the battlefield by Allied tactical planes meant the Germans had to give up lightning raids in the open and settle down to trench warfare, much like in World War I. The Allies remained free to move.