Andrew Langer proposes the absurd option of putting U.S. forces in Ukraine:

We took it upon ourselves to engage in action in Vietnam, Iraq, and Afghanistan — no treaty or agreement forced our hand. But in the Ukraine–Russia conflict, we may very well have a legal obligation to take action. In order to induce Ukraine to give up its nuclear weapons following the dissolution of the Soviet Union, we pledged in the Budapest Memorandum to assist them should they face the kind of threat they are facing right now. There is genuine debate about the exact meaning of the memorandum as it relates to the current scenario, but at the very least, America’s inaction calls into question our credibility and our commitment to preventing the proliferation of nuclear weapons.

Let’s consider all the ways that this is wrong and dangerous. The U.S. has no legal obligation to come to Ukraine’s defense. The Budapest Memorandum was never ratified, it is not legally binding, and even if it were its provisions do not include anything remotely approaching a security guarantee. This has been addressed many times already. There is not really any argument about what the memorandum requires from the U.S. and U.K., and the only people that believe that it requires military assistance are those that don’t know what the memorandum says or pretend that it says something that it clearly doesn’t. So Langer is simply wrong on this point.

The U.S. isn’t obliged to defend Ukraine, but would it still be the appropriate thing for the U.S. to do anyway? That is even less persuasive. Putting American forces in Ukraine now would put them in an extremely difficult position. Their presence would alarm Moscow and in all likelihood trigger the conflict that their presence is supposed to discourage. Sending them there would perversely risk further escalation in the form of direct and open Russian intervention. That would then leave U.S. forces with the unpalatable option of trying to expel Russian forces, which creates the danger of a general war with a major nuclear power that even Langer finds unacceptable, or withdrawing in the face of a Russian advance that would render the entire exercise useless and embarrassing.