Dec 8,1941, after Japan attacked the US. But we pretty much helped them all along.
Allied powers* because they were attacked by Japan.
we didn't. the US joined the Allied powers.
America declared war on Germany April 6, 1917.
April 6 1917
In 1890.
In 1787.
1861
The United States of America joined the Allied Nations in World War II.
AnswerThe U.S. joined the allied troops, against Germany and their forces.
1941
1845
Congress declared war in July 1917 but it took till later in the year for US troops to get to the European war zone. The US joined as an "Associated Power" to avoid political entangelments with France and Britain.