Honestly I think it’s basically a combination of four things at once:
Separatists overreacting to news of civil war in Odessa and moving to declare independence from Ukraine
Kyiv blowing the separatists out of proportion and sending its army in a protracted siege war against them instead of sitting down based on Minsk agreements
Putin blowing the “fascist Kyiv government genociding Russians” thing out of proportion and sending his army in a blitzkrieg to destroy Ukrainian capabilities instead of sitting down with Zelenskiy’s government at the highest levels
NATO ignoring Russia’s repeated concerns over the years about expansion and surrounding it with the same kind of thing that made JFK ready to go to war during the Cuban missile crisis
Basically on every level, sending an army backfires. Better to just not react with violence and use diplomacy. But seems that cooler heads do NOT prevail