On September 4, 1941, after the U- 652’s torpedo missed, the USS Greer retaliated with an unsuccessful depth charge attack against the U-Boat. This first overt act of war was interpreted differently by Germany and the USA.
The USS Greer, built in WWI, was similar in appearance to fifty old destroyers given to the Royal Navy by the U.S. in 1940. The Germans also claimed that the U-Boat believed bombs dropped by a British airplane were depth charges from the destroyer.
On September 11, 1941, FDR gave U.S. Navy patrols permission to “shoot on sight” when encountering German submarines.