During World War II, the United States originally requested to remain neutral, only to get involved if they could gain something out of it.
On December 7th, 1941, the Imperial Japanese Navy Air Service orchestrated a surprise air strike on upon the United States against the U.S. naval base at Pearl Harbor in Honolulu, Territory of Hawaii.
The attack was intended as a preventative action, to neutralise the US navy and prevent the US from interfering with their planned military actions in the Pacific theatre. Curiously enough, all of the United States' aircraft carriers were untouched, as they had been out on a training mission, when the Japanese attack happened. The Japanese also neglected to target the naval repair yards oil tank farms, submarine base, and old headquarters building. This allowed the US to quickly repair and return to service many of their damaged ships.
As a result, this unprovoked attack by the Japanese on a neutral country, prompted the United States to declare war on Japan. Since Japan was allied with Germany, and Germany was at war with many of the United States' allies from world war 1, unofficially, the United States would already be at war with Germany, but on December 11th, 1941, just days after the bombing of Pearl Harbour, the United States Congress officially declared war on Germany.