“What’s past is prologue”
Your historical ignorance does not serve you well. Putin is trying to reassemble the Russian empire because whether a Czar, or Stalin or Putin, Russia has always felt a historical entitlement to rule over all the Slavs.
For the moment we might appease Putin by abandoning Ukraine. Are you naive enough to think he’ll stop there? I was just in Slovakia which is a much smaller country just south of Ukraine. They’re nervously looking north imagining the Russian military just on the other side of the border. Where does it stop?
China is also a threat that must be dealt with. US weakness invites aggression whether from China or Russia.
I have likely been reading history longer than you have been alive. One aspect that is more than obvious, is that a desire to form an empire does not equal the means to form an empire. You ask how Putin will be stopped, when he has already been stopped by the less-than-mighty Ukrainian military; Russian conventional forces that have not taken Kiev after almost three years are no threat to NATO.
Rather than ask how Putin will be stopped, why not ask how he might accomplish the conquest of Europe that you fear? He is massively out-gunned by the long-term members of NATO; in addition, every Warsaw Pact country that supported the former Soviet Union is now a member of the western alliance. Do you fear that the mighty PootyPoot will appear on the battlefield in person, riding a unicorn and shooting death rays from his eyes, to triumph over NATO (with appropriate victory parades through Piccadilly and the Arc de Triomphe)? The Russian army sure can't do it.
Russia has a military budget comparable to Britain or France (not the US, and not even China); and Russian conventional forces are far less competent (not more competent) than western forces. Your willingness to ignore simple facts does not serve you well...