In 2021, Russia was on the verge of complete political and economic control of Germany, and by blocking pipelines from the middle east, had made much of Europe dependent on them for energy. NATO was dying on the vine, its military spending in a 3 decade decline. A quick, decisive war with Ukraine probably would have caused the collapse of NATO, and returned Russia to Great Power status. With a leadership vacuum in the White House, it was an incredible opportunity that Putin couldn't pass up. He just underestimated Ukraine's ability and willingness to fight. Trump warned them during his first term to break their energy dependence and spend on defense. They didn't listen, even after the first invasion/occupation/annexation in 2014. Weakness invites aggression. Si vis pacem, para bellum.
And the extent of the corruption in the Russian military. All of his Generals were stealing and they did not spend $$$ on the military. They had a 40 mile convoy that was supposed to seize Kyiv on Day One. Remember?
Instead, NATO is re-invigorated, Russia's potential economic/energy stranglehold over Europe is over, and Russia's long-term northern geopolitical strategy with the Nordics has gone horribly off the rails. Instead of Finland remaining cowed and Sweden staying neutral, both joined NATO.