American logistics in the Northern France campaign played a key role in the Allied invasion of northwest Europe during World War II. In the first seven weeks after D-Day, the Allied advance was slower than anticipated in the Operation Overlord plan because the well-handled and determined German opposition exploited the defensive value of the Normandy bocage country. The Northern France campaign officially commenced on 25 July, the day First United States Army began Operation Cobra, which saw the breakout from the Normandy lodgment, and ended on 14 September.