Україна Ukraine

Russia has already proven they won't stop with just part of Ukraine. They need the whole country.

Ukraine isn't even their objective... it's just on the way to their objective.

Every post-Soviet invasion that Russia has launched has been about re-establishing control over the geographic choke-points that Russia has desired for centuries, which would create borders that can be guarded by a relatively small force in a few key positions defined by impassable terrain, enabling their indefensible flat interior lands to be efficiently defended. Imperial Russia tried to expand for centuries to reach these points, gaining some of them, but it was only under the USSR that Russia actually managed to accomplish it, controlling all of the easy access points that an invading army could use to get into Russia. And then they lost all but one of them in 1991.

Two of them are just beyond the western and southern borders of Ukraine, in what have now become NATO countries.

With Russia's failing demographics (they only have about 6 million men aged in their 20s, and the following generation is even smaller), they are in a "build secure borders now or we'll never be able to do it" situation. They literally can't stop with just part of Ukraine's territory... that leaves them worse off, with longer and less-defensible borders than which they started, and with fresh new enemies.

Any "peace" deal would only be a very temporary cease-fire.