If the US takes the focus "Unholy Alliance", and the Soviets accept them into the faction, the US is granted a decision that, when the USSR is at war, allows them to annex the USSR if the USSR ever gets a surrender threshold of 20% or lower as long as the US is in the ComIntern (the US needs to be communist if it wishes to join the ComIntern by clicking the "Join Faction" button, but it can take the focus even if it's democratic).
Note that this decision does not core the USSR, it only occupies it (likely due to how overpowered said decision would be), although the US does not need to be in the same war as the USSR to take the decision, meaning that if one was to use the decision while the USSR, the nation that had been at war with the USSR would be forced to leave the territory that had previously belonged to the USSR.