The US is a democracy that decided as a democracy to invade Iraq. That turned into a shit-show. It isn't necessary to delve any deeper into those two facts to recognize that appeals to what democracies decide are insufficient.
If the US invaded Canada, Canadians could righteously defend ourselves. If there isn't an obvious practical path to success, what would be the point? What are the differences between governance from Ottawa or Washington that would be worth dying for; more importantly, what are the differences that would justify compelling others to risk death or lesser injury with unlikely prospects for success? Now shift the frame to Ukraine and Russia. For each person who considers the questions, the answers might be different - there is always going to be some threshold at which the differences are intolerable. But try to keep emotion out of the analysis.