Sinking of the Lusitania and breaking promises of un-restricted submarine warfare. Preventing U.S trade.
The US did not declare war on Germany in 1941, Germany declared war on the US as a measure of solidarity with Japan after their attack on Pearl Harbor and the Philippines.
Germany declared war on the United States on 11 December 1941 and the US responded accordingly.
The U.S. declaration of war on Germany in December 1941 was primarily precipitated by the Japanese attack on Pearl Harbor on December 7, which led to the U.S. entering World War II. Following this attack, Germany declared war on the United States on December 11, 1941, prompting the U.S. to respond in kind. Additionally, the growing threat of Axis powers and Germany's aggressive military campaigns in Europe contributed to the urgency for the U.S. to formally engage in the conflict.
The German telegram to Mexico and the sinking of U.S. merchant ships.
There has been military conflict between USA and Germany in two wars. In WWI it was USA declaring war on 6 April 1917. In WWII Germany declared war on 11 Dec 1941 as a response on USA's war declaration on their ally Japan on 8 Dec.
Because of the Japanese attack on Pearl Harbour in December 1941- Japan and Germany were allied by treaty, and the USA could not go to war against Japan without declaring war on Germany as well.
America declaring war on Nazi Germany and joining the Second World War in Europe.
11 DECEMBER 1941 http://www.historyplace.com/worldwar2/timeline/germany-declares.htm Y-THINK-Y
11 December 1941
Japan attacked the US without Germany even knowing about it. Germany declared war on the US on December 11, 1941. Japan probably didn't even know about that until it happened. Hitler had invaded Russia in 1941 and no doubt was hoping Japan would open a second front on the Russians as a favor for him declaring war on America.
mobilization
it became global