I'm not so sure about a firm security guarantee because I really don't think it's worth going to war with Russia over Ukraine. We never should have been at this place in the first place because there was no reason to go all out with Russia over the EU association agreement. That's an important point. However, we are in the mess now, and unfortunately we have to do something because if Russia actually moved troops into Ukraine and there was no response, it could give the impression of weakness. Especially after the heavy handed US incursion into Iraq, a thus embolden Russia could feel the need for other incursions elsewhere to demonstrate that it's status on the world stage is equal to that of the US. But over Ukraine, I personally would do no more that impose some very harsh sanctions and possibly send some weapons. Although with the weapons, I would have to think deeply because there are a lot of right wing, militant extremists who are a part of the government of Ukraine right now and there is no telling what they might do with sophisticated weaponry. I think the proper place to draw security guarantee line right now is NATO members.
That said, what is troubling currently is that Russia says that mercenaries from US security companies are operating with nationalist militants from Right Sector and Ukrainian security forces in Eastern Ukraine. If that is true, then Russia could perceive that the US, through mercenaries, could be attempting to infiltrate Russia itself through Ukraine to provoke unrest and destabilize the Russian government. That would not be an unfounded conclusion because indeed the US did use mercenaries in Afghanistan, and I think is is highly likely that those mercenaries were used to help facilitate and encourage those groups in Afghanistan such as the TTP that were carrying out attacks in Pakistan aimed at destabilizing the government. The President of Pakistan, Asif Zadari, husband of the slain Benazir Bhutto, told US envoy Zalmay Khalizad that he believed the US was behind a number of attacks inside of Pakistan. He said the purpose of the attacks was to destabilize Pakistan so that the US could seize it's nuclear weapons. So it's possible that US mercenaries could infiltrate Russia through Ukraine and foment unrest in Russia to destabilize Putin's regime. We are likely already doing so through NGOs in Russia, so while it's not a known fact, such a scenario is a possibility. Not only that, but right wing extremists like the Right Sector could even carry out terrorists attacks in Russia using Ukraine as a staging ground. IF that's the case, and Putin senses this as a threat, then it would become highly likely that he would send troops into Ukraine, thus escalating tensions. If we had a firm security guarantee with Ukraine, this would be very unfortunate because direct confrontation between Russian troops and US military personnel would become likely. That would be very, very unfortunate.
Now one could say that's all the more reason to give a security guarantee to Ukraine because that would reduce the possibility of a Russian incursion. But I think that given the very close proximity of Ukraine to Russia and the close historical ties the Russia has with the people in eastern Ukraine, we would be playing a very dangerous game of Russian roulette, pun intended, and I see no good reason to take such a risk over Ukraine. Again, the countries of NATO is the proper place to draw that line.