It's not clear that Russia actually wants to take over the entirety of Ukraine. It seems pretty unlikely, based on how much of a nightmare it would be to take and hold. Taking over the parts of it closest to Russia and which are most supportive of Russian rule, sure. But at that point Ukraine isn't faced with complete annihilation, just losing some of it's territory.
Don't do this. Diplomacy is always an option. Fighting is only "mandatory" when it's that or death, and even then you can choose death. Making it sound like war is inevitable is just making excuses for the emotional response you want to have to the situation.
If Putin has managed to manoeuvre Russia into a geopolitical situation in which the best outcome for everyone is to cede the territory, then that's what should be done. There are times in chess when you get outplayed, and your choices are to sacrifice a pawn, a knight or a queen. You can attack if you want, but it just results in a worse situation than accepting the reality and playing for the best overall outcome.
This would suck for a whole lot of reasons and maybe it feels better to people on the outside to bang the drum and send the troops to war, but they're not the soldiers and civilians who are going to be displaced or killed on the ground. You shouldn't go to war just because you're angry.
It's entirely possible that the West is simply going to have to admit that they have been outplayed again, and that there's no reasonable way for them to "win" this round. They can chuck a tanty and send in a bunch of people to die, or they can make a plan to avoid ever ending up in this situation in the first place.
