Answer:
1939 ⇒ Sales of weapons to Allies on Cash and Carry basis.
The U.S. was still neutral in 1939 when the war broke out but they did allow the British and other allied powers to purchase weapons from the U.S. provided they paid for it there and then and took it.
1940 ⇒ Exchange of Destroyers for British bases.
In 1940 with the British low on military resources, the Americans exchanged 50 US destroyers with the British on the condition that they allowed them to use British military basis in Newfoundland and the Caribbean for 99 years.
1941:
- Lend Lease Act
- Officially entered Second World War
The U.S. passed the Lend Lease Act in 1941 which allowed them to supply the Allies with the weapons and resources necessary to win the war. Without this Act, the Western allies would most probably have been beaten.
This was also the year the U.S. officially entered the war by declaring war on Japan on December 8th after the attack on Pearl Harbor. They also declared war on Germany and Italy on December 11 in response to the two nations doing so first.
