> Half-measures don't work in war.

Peace talks are half-measures too though. Ukraine will have to agree to give up Crimea and Donbas, and not allow NATO troops in, then Russia will regroup, retrain, and re-arm, then try again to capture all the territory east of the Dnieper. After all, having proven that the only cost of invasion is temporary sanctions, why wouldn't Putin try again?

If half-measures are ruled out, the only available extremes are complete capitulation (effectively annexation of Ukraine, under a puppet regime, similar to Belarus) or nuclear war, which is what it would mean for NATO to "fully engage". For a non-suicidal listener, then, a call to avoid half-measures is a demand to surrender, hidden behind an olive branch.