I think Russia will defeat Ukraine - if NATO doesn't send troops. If NATO sends troops, I think it will end like Korea.

If NATO doesn't send troops I don't think it will play out like you say with troops at the border of Poland. That would galvanize the West and NATO together.

Russia has been too savvy, with the exception of NOT doing more between 2004 and 2022 to avoid being put in the position of Ukraine entering NATO and invading Ukraine. That being said, 2000 Russia was very weak after the 90s under Yeltsin and maybe nothing more was possible.

If NATO doesn't enter and Russia continues to destroy Ukraine, I think they'll stop there to prove to the world that Putin is not Hitler. Then offer to reopen Nordstream II after rebuilding with additional security.

THAT situation would put NATO under more stress. Russia just saying, "OK, can we stop these sanctions now before Germany's industrial base is completely destroyed forever?" By that time Germany will be under extreme political pressure domestically to do something to save jobs lost to very expensive industry.