Well, if the US doesnt send lend lease to the UK and the USSR, the soviets and UK loose an massive advantage. Overall, the soviet army lacked tons of trucks for example, which the US provided via lend lease. Without the US Airforce things would have looked different aswell. You cant really say what would happen because there are tons of factors that influence the outcome, but the allies would have had an much harder time for sure. In the worst case the whole of europe would have been communist or facist. The end result would have been similar tho, in that either the communsits or facists clash against the US eventually, even tho an invasion would have been practically impossible for the US since they would have lost the bridgehead of the UK.
One thing is for sure, the US wouldnt have been where they were after the war without all the captured german technology and scientists.