copy/pasta from wikipedia, that's a bad brit!
The US kept england afloat even during the battle of britain thru raw materials and logistical support. If britain didn't have the tons of supplies thru the US (before the latter officially entered the war), the brits wouldn't have made it.
Fun fact - when the germans switched from military targets (specifically radar) to bombing cities, the british fighter command was within 24 hours of defeat themselves. the germans, of course, didn't know that ... the rest is history