Donetsk and Luhansk have declared independence, and Russia has recognised them. Secession is not a new phenomenon; Texas seceded from Mexico. Several regions seceded from the former Yugoslavia, and were rapidly recognised (and reinforced militarily) by western countries.

I think it's perfectly legitimate to argue that Russia is not invading Ukraine, but rather supporting the Donbas republics. I mean, I don't agree; and I think there's a fair chance that Russia will invade Ukraine, and install a puppet. But they haven't done it yet. I suspect that for the next few weeks, they'll restrict themselves to air and missile attacks on Ukrainian military installations, with troops-on-the-ground restricted to Donbas.

Beyond a few weeks, who's to say? No plan survives first contact with the enemy.