US joined the allied powers in April 6, 1917.
Step-by-step explanation:
The allies were Britain, Russia and France to take part in World war-I. Two million plus US soldiers fought on war fields in France under Major General John J. Pershing's command.
On April 2, 1917, President Woodrow Wilson attended a meeting and declared a war with Germany. Wilson spoke of Germany's breach of his promise to suspend the underwater war indefinitely in the Mediterranean and North Atlantic, as well as his attempt to lure Mexico into an alliance with the US, as a result of his declaration of war.
On April 4, 1917, the US Government voted to enact measures to declare war in the United States. The house was approved 2 days later. The US then declared war on December 7, 1917, against its German ally, Hungary-Austria.