Germany declared war on the U.S. after Japan attacked Pearl Harbor
Internal reason :
The United States of America is located far from the rest of the world
(on another continent), so the war never had a chance to occurred in the
States. But as the Axis had taken/occupied nearly all over Asia and
most parts of Europe, the States found that sooner or later, when the
Axis had conquered all of these countries, the sword would pointed to
it. So, the president had laid an order to the Navy that he wanted the
U.S. ships to patrol a "hot" area to have immediate response if things
got worse.
External reason:
Aware the existence of U.S. naval ships at their water, the Empire of
Japan decided to take action by attacking Pearl Harbor. As some sources
(not reliable) stated that the U.S. had noticed the preparing actions of
the Japanese but they ignored it, so the attack caused heavy casualties
to the troops, that would ring the war bell from the citizens and that
would make the perfect cause for the US to joined the Ally and
officially enter World War II .
