There were mistakes leading up to this, but the second Khrushchev become general secretary instead of Malenkov is when the union became doomed to fail.
>focused on military dick measuring with the west instead of civilian production
>recriminalized the orthodox church
>entire reason for the sino-soviet split
>liberalized/decentralized the economy without investing into automation/technocracy so the union slowly stagnated
>virgin lands initiative mixed with lysenkoism instead of just giving incentives to peasants
It's hard to imagine any other point at which the USSR was made to lie low than when corn man was "chosen" (took power in a coup) to lead it.