In WW I, when we finally got into it, I think our main contribution was to discard the "war of attrition - dig more trenches" mentality, that held sway for the previous four years. That had the effect of bringing the war to a conclusion, instead of going on for who knows how long.
WW II, however, was quite different. Churchill did a great job of convincing the British people that grit/ pluck/ stiff upper lip, would prevail. So much so, the Brits that lived through it, carried that belief with them to this day.
I believe England would have fallen to the Nazis before we could ramp up our military machine, without the support and material we were providing them.
Bare handed pluck = martyr.......... Pluck, with US supplied material = survivor.
It's pretty hard to deny the US was the big dog, the decisive factor, in determining the outcome of WW II. But, that said, if Britain owes us for our WW II efforts, we owe them for holding the line while we were ramping up, because the Nazis were after World domination, not just Europe.
Thank you.