I'm afraid if the United States leaves the invasion of Ukraine unpunished, it will embolden China to try Taiwan, embolden Putin to continue annexing former Soviet territories and give the Iranians enough confidence not only to develop a nuclear weapon, but put it on top of a shiny missile.
You mix all that ugliness with a weaker United States and a global recession, that's a recipe for a World War.
This is one of those nip it in the bud events.
FWIW, I don't think Putin would go to war over Ukraine if push came to shove. Even a limited engagement with the US would kill the global economy and namely the golden goose of Gazprom. I think he's merely trading on his calculation that Obama goes weak in the knees when action is required. If say Reagan, Bush or Clinton were in office, this would have never happened. Reagan would blockade, Bush would invade something, and Clinton would have carpeted the area with cruise missiles for three weeks.