Technically, they didn't start it. They just allowed it to happen by not backing down after Putin sperging about it for years, with the idea that it would topple Putin's regime due to the attrition... With the hope of getting a pro-Western regime. The US javellins etc were already present after all.

I wonder how much Ukrainian blood would need to spill before they decide that continuing the fight won't be worth it anymore.

Anyway, for being an obvious proxy war, a lot of people still aren't aware of that.