“If Ukrainian forces attempted to retake the territory in eastern Ukraine (Donbas) held by pro-Russia separatist rebels, he warned, the Russian military could very well declare war and march on the Ukrainian capital of Kiev, whatever the costs. He also described potential dangers from the military buildup in Europe — particularly American missile defense there — and discussed when Russia’s nuclear doctrine would or would not allow the use of nuclear weapons.”
Source: Former top Russian general: Russia will defend eastern Ukraine, even if it means taking Kiev – Vox
