The news of the attack on Pearl Harbor put Hitler in a "euphoric mood" and that Hitler "had an ally that had not been conquered in 3,000 years." Berlin saw the Lend Lease Act and Americas actions in escalating the battle for the Atlantic as a virtual deceleration of war on Germany. Hitler wanted a more aggressive attitude in Japan in order to strike a blow against British colonies but also to distract America. With the attack on Pearl Harbor and the subsequent declaration of war on America by Germany, Hitler believed that the US would be forced to tie down most of there forces in the Pacific while also being forced to keep supplying the Western Allies via the Atlantic would force America into a two front war. On December 8th, Hitler allowed his U-Boats to now attack US shipping which would, in turn, weaken British power against the Japanese. All this would now give Germany time to focus strictly on the Soviet Union before the inevitable land war with America. Hitler obviously underestimated the power of the US and over estimated the power of Japan. Hitlers fate was now sealed but it did buy German time before there ultimate failure in November on 1942 with the Stalingrad catastrophe and the invasion of North Africa.

Source: Stephen Fritz- Ostkrieg: Hitlers Was of Extermination in the East

You have to remember that since the passage of the Lend-Lease Act in March 1941 the US had effectively thrown its hat in Britain's ring. Part of the lend-lease was sending supplies to Britain and in the respect the US Merchant Marine was taking part in the Battle of the Atlantic for months before any official declaration of war.

From anyone's perspective the US was no longer neutral. It had started exclusively providing war material to the UK and later to the USSR so even without Hitler formally declaring war in December 1941, formal hostilities between the US and Germany were inevitable.