As the war in Europe progressed during 1939-1941, the US policy changed from neutrality to supplying essential military aid and other assistance to the allied nations to support them in the war.
Explanation:
The President of the U.S Franklin D. Roosevelt made it clear before the second world war that the U.S would not be taking part in any wars in any degree of involvement thereafter.
But as the Second World battle began, the United States began supplying essential military aid and other necessary assistance to its allies.