Did Germany attack American ships before the US declared its involvement in World War 2?

Foolishly, yes, they did, December 1941.

It has never been made clear why but Hitler/Germany did declare war on the USA at this time, right after Pearl Harbor was attacked by the Japanese. That attack, of course, led the USA to declare war on Japan.

Possible reasons Germany engaged the USA:

1) They felt they were already at war. The USA was actively arming and financing German enemies. The USA had attacked many German naval vessels and even murdered some helpless German sailors.

2) Germany hoped that by going against the USA they could get Japan to attack the Soviets in Siberia. No such luck though, as the Japanese were completely outclassed on land to fight a major European army and they knew it.

3) Hitler may have thought he had already won the European war and might as well get into a fight with the USA too. It appeared at the time that Moscow would fall to German forces, which would have crippled the Soviet chances for victory.

Thus Germany would only need to sit and defend their European empire from the west, a relatively easy situation. In other Words, Hitler might have thought that the USA involvement in Europe would be of little consequence and a declaration of war was just a political statement.

In retrospect it would have been smarter for Germany to hold off on a declaration. Instead they should have worked to stabilize the eastern front with Russia and build up western defenses. They needed to wait and see what happens between Japan and the USA. With an active war, the USA would probably have pulled back some support for Britain/Russia and put all their military effort into the fight in the Pacific.