I think we should make one thing clear. Despite how the President misspoke last Thursday, a tragedy has occurred and is occurring in Ukraine. Forget geopolitics - there is not anything that could be said that could write off the loss of human life that has been experienced in Ukraine.
Still a tragedy is not the only necessary ingredient to justify United States involvement in an international conflict. Sure, if we were talking about people, a tragedy would instill an obligation for anyone with the capacity to help to assist in every way they can.
But government is not a person. Further, government is not accountable as a moral agent, the same way that we as human beings are. So we cannot necessarily use the same logic and moral rationale that we would for people when we are discussing government decisionmaking.
Government instead is an entity created by people of a nation for a specific purpose. Generally considered the social contract theory, the people of a nation come together to form an entity designed to protect them from natural disasters (not to help clean up the damage from a natural disaster though), external threats to national security, and internal threats to the rights of the citizens (i.e. criminals). Any action taken outside this purpose is a violation of a contract.
Consider this analogy, quoted here from my prior post "Not Yours to Give: Unconstitutionality and Injustice of Redistribution,"
"When we hire a business to put in a home security system, we would be much annoyed to say the least if we heard the company had decided instead to use our money to redecorate our house, as they considered that a wiser use of our money."
I daresay it wouldn't matter to us nary a bit if we still got our home security system if they took more money from our bank accounts to buy us new furniture or generally use for purposes other than that which we have contracted them. With that in mind, we ought to consider whether the situation in Ukraine is actually considered a part of our contract.
I think it is not too much of a stretch to say that the situation in Ukraine is not likely to affect our national security. If the situation proves that it will escalate from a simple border conflict and actually harm United States situation, then obviously we must act.
That is actually what I believe President Obama was trying to say when he said that a "tragedy may have occurred." And I must give him credit that he understands that not every geopolitical conflict requires his involvement (I obviously don't have to give him credit for his skills in communication).
But that might raise a question in your minds. Why should it matter that the security of our nation isn't at stake? Why can't our government act in charity to assist other nations in their struggles? Is not Ukrainian life worth as much as American life?
The answer to the latter question is obviously yes. But we must once again realize that governments are not moral agents, and should never participate in charity, especially on an international level. Let me remind you of the words of Horatio Bunce summarized by Davy Crockett in his speech "Not Yours To Give," Note that while this specifically pertains to money, the same principle would refer to any type of military involvement.
"The power of collecting and disbursing money at pleasure is the most dangerous power that can be intrusted [sic] to man, particularly under our system of collecting revenue by a tariff, which reaches every man in the country, no matter how poor he may be... So you see, that while you are contributing to relieve one, you are drawing it from thousands who are even worse off than he. If you had the right to give anything, the amount was simply a matter of discretion with you, and you had as much right to give $20,000,000 as $20,000. If you have the right to give to one, you have the right to give to all; and, as the Constitution neither defines charity nor stipulates the amount, you are at liberty to give to any and everything which you may believe, or profess to believe, is a charity, and to any amount you may think proper. You will very easily perceive what a wide door this would open for fraud and corruption and favoritism, on the one hand, and for robbing the people on the other. No, Colonel, Congress has no right to give charity."
So yes, a tragedy has happened, but no, the United States government, since it is not a moral agent is under no obligation to act. Indeed, it is under a strict obligation to remain neutral in this particular affair.