1. A negotiated agreement...

2. A failed state: low intensity war, low cost financially and diplomatically. Yes having a failed state next door is bad but how about having THAAD, nuclear tipped cruise missiles or intermediate range nuclear missiles next door. With a failed state, it is too dangerous for both the US and Russia to station any nuclear weapons in Ukraine. What if the leader of a failed state decides to take control of any nuclear weapons stationed on its soil? Whose fault is it and would Russia give the US an easy pass for the fallout (are you serious)? What if a terrorist group takes control of the nuclear weapons? No one is stupid enough to station nuclear weapons in a failed state and having nuclear weapons next door is Russia's worst nightmare. Trust me if given a choice a smart leader would choose a failed state over a full fledged NATO member state.

3. A full invasion and regime change: we see the consequences...do you want to be a Russian citizen (not just an oligarch) now? And after all the blood and casualties, do you expect the Russians to just pull out and leave the current Ukrainian government alone? What is the end game? The very least: they would set up a puppet regime or force out the current government but long term occupation of Ukraine or re-absorption of Ukraine is too costly in terms of treasure, manpower and international diplomacy. If a permanent puppet regime is not possible, would there be a repeat of the current situation?