Bigun1_6605 - although you are correct with your accounts of military engagements between US and German personnel prior to 1942, the term "troops" generally signifies ground forces rather than those of the Navy (or Air Force).
This is an extract from the German Declaration of War against the US - December 11, 1941...
"On September 11, 1941, the President of the United States publicly declared that he had ordered the American Navy and Air Force to shoot on sight at any German war vessel. In his speech of October 27, 1941, he once more expressly affirmed that this order was in force. Acting under this order, vessels of the American Navy, since early September 1941, have systematically attacked German naval forces. Thus, American destroyers, as for instance the Greer, the Kearny and the Reuben James, have opened fire on German submarines according to plan. The Secretary of the American Navy, Mr. Knox, himself confirmed that American destroyers attacked German submarines.
Furthermore, the naval forces of the United States, under order of their Government and contrary to international law have treated and seized German merchant vessels on the high seas as enemy ships."