My sense is that however the Ukraine war ends.... Unfortunately, I think it's likely to end with Russian troops still occupying some portion of Ukrainian territory. I listened to the first part of this session. This was also a theme. My sense is that however it ends, the only way to prevent Russia from restarting that war as soon as it's rebuilt its military capability to what it considers the required level is for Ukraine to have legally enforceable security guarantees. Whether it's NATO membership or some consortium of countries—probably almost all NATO members—they would give Ukraine legally binding security guarantees and say, “If you are invaded again by the Russians, we will fight.”
This then becomes the question: How do you deter that invasion? Is a legal document, whether NATO's article 5 or some other legal document, adequate? Do there have to be forces on the ground? Does there have to be a deterrent force from non-Ukrainian militaries present in Ukraine?
I think the latter is far more likely to deter renewed aggression than just binding security guarantees, but it also comes at much higher risk, because then immediately, as we already have in eight frontline NATO states now where there are battle groups present, a Russian invasion of any of those countries, or of Ukraine if there are forces on the ground, would put western military forces in a state of war with the Russian Federation.