World War III didn't just start, but the first major land war in Europe since the end of WWII did. It is scary for what it portends. If the West doesn't stand up to this aggression by at the very least turning Russia into a pariah state like North Korea and arming the Ukrainians to the teeth with every spare bullet we have; then it won't end with Ukraine.
The US (and NATO) will not, and arguably should not, fight on Ukrainian territory. But if the aforementioned actions are not taken, then Latvia, Lithuania, and Estonia are next on Putin's list; and they are NATO members. And if the Alliance doesn't stand united in supporting Ukraine in resisting Russian aggression, then Putin will surmise that they will not stand united in their collective defense and Article 5 of the North Atlantic Treaty isn't worth the paper its written on.
Putin has said since 1999 that he thinks the collapse of the Soviet Union was the greatest geopolitical tragedy of the 20th century and he longs to reclaim not just its territory, but its influence. Remember, Soviet influence in Europe wasn't just former Republics of the Union of Soviet Socialist Republics, but also the Warsaw Pact; whose western border was a wall down the center of Berlin.
And all of this isn't even taking into account our primary geopolitical foe in 2022, the Peoples Republic of China. How the US and the Western world responds to events in Ukraine will undoubtedly inform China's decision to move against Taiwan and further actions in the South China Sea.