It would however be triggered by sending troops into Ukraine and then advancing those troops or firing at troops inside Russian borders. Which the only way for Ukraine to win, considering how outnumbered they are, is to take territory in retaliation to invasion.
When one side is free to take your land and you are forbbiden by your allies to take their, well your hands are tied and when the other country is 1000x your size it is a guaranteed loss for you eventually.