Well, US/UK did it once with USSR. They allowed SU control over Poland and east Germany.
You have forgotten the Cold War. The Russians stopped only where they were forced to stop. Western European countries set up an entire new international organization, NATO, for cooperation in case of a Russian attack on any of them, and permanently maintained massive armies to until the very end of the Soviet Union to prevent any further Russian creep west.
The US, UK, and others did not pack things up and go home at the end of WWII, believing that the Russians had their belly full with Eastern Europe and wouldn't push for more. The UK, for example, withdrew its last forces from continental Europe only in 2010. The US withdrew last combat forces in 2013.
Looks like Russia took that as an invitation to invade Ukraine the very next year.
The US, UK, and others did not pack things up and go home at the end of WWII, believing that the Russians had their belly full with Eastern Europe and wouldn't push for more. The UK, for example, withdrew its last forces from continental Europe only in 2010. The US withdrew last combat forces in 2013.
Looks like Russia took that as an invitation to invade Ukraine the very next year.