Question

How did the Destroyers for bases agreement President Roosevelt signed help Britain and its allies

Answer 1

The agreement put US bases on British territory.

Answer 2

#1: Britain was able to get more destroyers to protect their convoys that were getting supplies from the US

#2: For the future (and yes they did use them later on), the US was able to get ready for the war and produce strike points (the bases) and build up their military from there

hope this helps