The united united states became involved in the war with germany because?

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 .

Because Germany declared war on United states after japan attacked pearl harbor united states declared on japan which made Germany declare on united states.