Question

How did the U.S. help Britain During World War 2?

Answer 1

Lend Lease and allowing British Security Co-ordination to operate in the US before the Americans entered the war. The US Armed forces after entering the War.

Explanation:

Lend Lease allowed Britain to trade overseas territories to the United States in return for 50 older destroyers and other considerations. Other countries were included as the War progressed including China and the Soviet Union. Enormous amounts of supplies and equipment were produced by the US and shipped around the world. Over a 3rd of the Trucks in the Soviet Union were American made by the end of the War.

https://en.wikipedia.org/wiki/Lend-Lease

FDR also allowed a large group of British intelligence operatives to operate within the US with FBI support.
The British shared there substantial knowledge of Codes and code breaking as well as scientific information on nuclear and other scientific development (the Tizard Mission).

The Americans were also creating their own secret intelligence service based on the British example (the Office of Strategic Services, the precursor to the CIA).

This was all happening before the US entered the War. After entering the War the volume of supplies and equipment increased. Troops were also sent.

https://archive.org/details/WilliamStevensonAManCalledIntrepid