Fairly early in his Premiership Winston Churchill had concluded that, while Britain could fight on alone, there was little prospect of defeating Germany over the longer term without the United States joining the war. Although Britain was already enjoying much material support from America, the long term aim of the British government was to see the United States fully engaged in the war.
Therefore there was careful monitoring of the US position. It was still very far from being the case that they would inevitably join Britain in the fight against Germany. Some senior Americans already thought of Britain as a lost cause.
Lord Halifax, British Foreign Secretary considered the position of the United States following the signing of the the Tripartite Pact on September 28th, in a paper circulated to the War Cabinet on 2nd October:
The first reactions in the United States of America appear favourable from our point of view, but we must be prepared for the American people to realise now more clearly than hitherto that if they go to war with Germany in our defence they too will have to fight a war on two fronts. Up till now this danger has been slurred over.
Now that it has been forced on public attention by the new treaty many Americans may feel that, since they are in no position materially to assume such a double undertaking, it will be best for all concerned, including ourselves, that the United States should continue to play the part of a benevolent neutral instead of becoming a harassed belligerent. This development will have to be carefully watched and countered.
The value of the United States to us at present is as a purveyor of munitions and moral support. The Germans realise that as they cannot now hope to deprive us of American moral support, their aim must be to deprive us of munitions. They might well think that the most complete way of doing this would be to involve the United States in war with Japan; the next best to distract American attention from Europe.
The conclusion of the Three-Power Treaty may go some way to distract American attention from Europe and from British needs for arms, and will also offer a better chance than now exists for involving the United States in war with Japan by encouraging the Japanese hotheads.
See TNA CAB/66/12/28
For the moment direct assistance from the United States came from individuals who volunteered to serve in the British armed forces. Prominent amongst these were the US pilots who had volunteered to served in the RAF. By September 1940 it was decided to form them into their own Squadron. No. 71 (Eagle) Squadron RAF was formed on 19th September and spent the rest of the year training. It converted to Hurricanes at the end of the year and became operational in early 1941. Eventually, after the US joined the war, the three RAF Eagle Squadrons were absorbed into the USAAF in Britain.
Compilation of newsreel footage from 1940-1942: