“...I think the last thing this government did right was WWII. Everything since has been less intentional and much less successful.”
The UN began October 24, 1945.
Well, there is that but I don’t think it is the whole cause.
Yes, we won the war but lost the peace. At the end of the war we had absolute military superiority due to the fact we had the atomic bomb and no one else did. The USSR did not have deliverable atomics in 1950.
Rooselvelt and Truman could have dictated the terms of Europe to the Soviets. He did not. He in effect gave all of Eastern Europe to the Soviets and 45 years of cold war then happened. Vietnam and Korea would not have happened except for the ignorance of Truman and relative to Roosevelt it was either ignorance or collusion.