You defeat Russia before the war begins, by keeping them impoverished as they are today. So long as they retain their current system, they will remain that way perpetually. Putin was bailed out by the dollar induced boom in the price of commodities from 2002-2014, specifically oil; that party is over. He has no second trick up his sleeve, which is why he is now turning to invading his neighbors.

Today Russia's economy is 1/9th the size of the US economy. In just 15 years, the Russian economy will fall to 1/12th the size of the US economy.

When you are so far behind economically, and showing absolutely zero potential for progress, the long-term military threat you can pose (outside of nukes), becomes proportionally restrained. Russia is now a regional power, barely able to maintain its own mass, much less project power against something like NATO.

Russia's ability to project force outside of its borders, is dramatically lower than it was during Soviet times, and it's going to remain that way. There's no need to fight inside of Russia, when they can't fight outside of Russia.