On June 6, 1944, the Allies launched Operation Overlord, the largest, most hazardous amphibious assault in history. The objective: establishment of a lodgment area in Normandy from which the Allies could strike at the heart of Germany and destroy the German armed forces. Air supremacy over northwest Europe was an absolute prerequisite for the success of the invasion, and to achieve it the U.S. Army Air Forces launched two campaigns aimed at destroying the Germans' transportation advantages in the area. In the months and days leading up to the assault, the Army Air Forces ceaselessly bombed rail centers, rendering most of the railways in northern France and Belgium unusable. Once the actual invasion was underway, the Allies shifted to an interdiction campaign, using precise air attacks on critical transportation installations near the battlefield to neutralize Germany's efforts to resupply and reinforce their troops. This work makes use of many wartime records that had remained classified until recently.