Lend lease mechanized Soviet infantry. Without it they wouldn’t have been able to hold out and then effectively launch counter attacks in depth, let alone feed their armies
Bingo. This is the key factor. It’s not so much that we have stuff away at Yalta as those were the facts on the ground at that point.
Wanna keep the Soviets out of Central Europe? Want to keep them from stripping it bare for labor and resources which enhanced their strength for decades? Don’t give them all those trucks and keeps and locomotives. It’s really that simple. The logistics backbone we gave them enabled them to project power more than a short distance from their supply bases - the problem the Germans never cracked.