The allies had a huge number of supplies and arms on their side when they entered World War II. They also had air supremacy which was one of the major turning points in the war.
Stalin was dictator in Russia .
Quite simply put, they didn't. The U.S entered World War One, to late to actually make a significant difference.
Allies in World War II
The United States was one of the Allies along with the USSR and the British Empire.
The Allies defeated the Central Powers, but barely. If America hadn't entered the conflict the Triple-Entente would have lost.
One of the main reasons the US entered World War I on the side of the allies was because the Germans had sunk the Lusitania, a passenger ship containing many Americans.
The US transported supplies to the Allies using merchant and passenger ships carrying United States citizens. This was why the German U-boats attacked these ships and were one of the reasons the US entered the war.
Britan was one of New Zealand's allies in world war 2.
The USA entered World War One because of repeated unlimited attacks on shipping in the Atlantic Ocean. Once the US committed itself to help the Western allies, it was vital that success in the war would do a great deal to end it. Defeat was not an option.
If you are asking if Italy was part of the Allies in World War Two, the answer is no. However, if you want to find the allies of Italy, the main one was Germany.
The US entered WW 1 late in the war. It's presence began in April, 1917. The US troops and airforce planes sent to France helped the Allies to defeat Germany.