Russias President Vladimir Putin welcomes President Barack Obama at the...

Russias President Vladimir Putin welcomes President Barack Obama at the start of the G20 summit in Saint Petersburg on Sept. 5, 2013. Credit: Getty Images

Is the West about to go to war with Russia over the fate of Ukraine? The question should answer itself. I can't imagine many Americans or Europeans willingly spending "blood and treasure" to keep Moscow's mitts off of Kiev. So why, then, did President Obama publicly warn Vladimir Putin that armed aggression against Ukraine would lead to "consequences"? What "consequences" did Obama have in mind? To put it another way, what cache of consequences could the United States fling at Moscow that would make Putin (or any Russian leader) change his behavior, or alter his cost-benefit calculus, when it comes to Ukraine?

Putin may face a bad month or so in the world media - perhaps face some sanctions and other troubles - for moving tanks, planes and Russia's own brutal brigade of riot police to quash protesters, overthrow parliament and restore some version of the old regime. But in his mind, that's nothing compared with the prospect of losing Ukraine.

Putin, after all, has lamented the breakup of the Soviet Union as "the greatest geopolitical catastrophe" of the 20th century. He considers Ukraine to be a Russian "territory," not an independent nation (and said so to President George W. Bush in 2008). And the Crimean peninsula, which Nikita Khrushchev ceded to Ukraine in 1954, is Ukrainian in name only, and even then just barely. (Khrushchev didn't quite surrender the land but declared it an autonomous enclave.) The Russian Navy maintains an important fleet there; most of its people speak, and regard themselves as, Russian. In the ongoing crisis, Putin did send troops to seize Crimea - to the complaint of few locals.

Yes, Russia signed an accord guaranteeing Ukraine's borders, and Secretary of State John Kerry scored debater's points by noting that Putin couldn't very well insist on Syria's sovereignty while violating Ukraine's. None of this matters to Putin, nor would it have to any other Russian leader in memory. Putin could cite the Crimean people's pleas to restore order in their streets (not that they'd been teeming with disorder).

If the crisis persists, he could easily find someone in the eastern part of the Ukrainian mainland - which is largely pro-Russia - to issue similar pleas. "I'm not invading Ukraine," he could say, "I'm only answering the calls for fraternal assistance from citizens endangered by hooligans and terrorists." (Indeed, in his phone call with President Obama last week, Putin reserved the right to protect Russian interests in Crimea and eastern Ukraine.) Of course, these are rationalizations, not real reasons. Putin's principal motive, now in the Crimea and possibly later in eastern Ukraine, is to reassert Russian hegemony.

Is this horrendous? Yes. Is it a big surprise? No. What can we do in response? Not a whole lot - again, unless we want to go to war, which would be stupid. There are good reasons why even George W. Bush backed off (or at least stopped short of pursuing) a pledge to consider Ukraine for NATO membership.

First, calmer minds weighed the level of Western interests in Ukrainian independence against the cost of defending it in a pinch, and found the former coming up short. (A military alliance like NATO, in which an attack on one is seen as an attack against all, should mean something.)

Second, polls suggested that only a minority of Ukraine's citizens wanted to join this alliance; about 40 percent saw NATO as a threat.

In 1959, and again in 1961, when Khrushchev threatened to occupy West Berlin, presidents Dwight Eisenhower and John Kennedy called his bluff, and Khrushchev backed down. If Khrushchev had sent in tanks, the United States couldn't have staved them off. (Berlin was in the middle of East Germany, and at the time NATO had only small conventional armies.) But West Berlin was a key Cold War battleground, a symbol of freedom and the home to millions of people who had been promised American protection.

Eisenhower and Kennedy said that they were willing to go to nuclear war to keep West Berlin free - and Khrushchev believed them enough to back down, in part because, despite his belligerent claims to the contrary, he had almost no nuclear weapons of his own. (Kennedy actually ordered a top-secret study on whether a nuclear first strike against Soviet military targets was feasible; it turned out, it was.) Ukraine is not West Berlin. More to the point, Ukraine is much more important to Russia than it is to the United States or to any Western European nation.

Russia is on Ukraine's borders; Putin sees it (as, again, would any Russian leader) as a vital market, supplier and, most important, a buffer against Western encroachment. None of this implies support for Putin's position, politically, morally or otherwise. It merely describes the facts on the ground: the interests, the stakes and thus the risks and options on all sides.

Which leads back to the original question: Why did Obama publicly state that aggression in Ukraine would trigger "consequences"? Clearly he was telling Putin to recalculate the potential costs and benefits of an invasion. But Obama was ignoring a simple fact: Putin would incur almost any risk to avoid losing Ukraine. To put it another way: There are no consequences - none that the United States could credibly threaten - that would keep Putin from doing whatever it takes to hang on to Ukraine.

More often than not, Obama has acted like a foreign-policy realist in the five years of his presidency. In his public statements on Ukraine these past 24 hours, he has not. Rather, he has drawn another "red line" that the threatened party feels it's worth the risk to ignore.

Obama spoke last weekend with Putin for 90 minutes, and Kerry is presumably talking with Russian foreign minister Sergei Lavrov. Russian news agencies are also reporting that Yulia Tymoshenko, the former Ukrainian prime minister (and erstwhile rival of deposed Ukrainian president Viktor Yanukovych) is traveling to Moscow to speak with Putin. All of these figures, not least Tymoshenko, understand what Soviet strategists used to call the "correlation of forces" - the contending interests, risks, possibilities (and impossibilities) at his historic moment.

Neither side wants an escalation in violence and disorder. The question is how much each side is willing to accept an escalation in the pursuit of its vital interests. And in that equation, Obama and the EU do not hold strong hands.

Under such circumstances in a crisis of this potential magnitude, Obama should be looking for common interests. One such interest is ending the bloodshed. Even Putin couldn't want to send troops to the Ukrainian heartland. The Russian army is hardly in tip-top shape: It could probably mount an invasion, but who knows how long it could sustain an occupation, especially in the face of nationalist insurgents who have a fierce, even ancestral hatred of Russia.

Perhaps Obama could offer assurances that he won't offer Ukraine membership in NATO (that's not a live issue anyway), nor will he push to revive the plan for Ukraine to join the European Union. This latter pledge would be a big deal: The protests were set off when Yanukovich cancelled plans for a formal association with the EU, after Putin lured him back into Moscow's bed with a $15 billion aid program. In exchange for these assurances, Putin would call off his shock troops, recognize the Ukrainian parliament's ouster of Yanukovich (whom Putin never liked anyway), and allow Ukrainian elections to go ahead this May, perhaps under international observation. Obama could present the deal as a victory for democracy (the Ukrainian people will decide!). Putin could swallow the deal, believing that a pro-Russia candidate might win (legitimately or otherwise). In any event, the Ukrainian politicians will have been shown what Putin could do if they get out in front of their skis again.

Then again, maybe they're not talking in these terms. Putin could overplay his hand, not wanting to look weak in his backyard and perhaps believing that the Western leaders won't walk away from him altogether because they need his help in Syria and Iran. Or maybe Obama and Putin could come to terms, but protesters and police in Kiev and elsewhere might have different ideas about an equitable solution - thus re-escalating the violence. (Just a few days ago, remember, Yanukovich and the heads of three protest groups shook hands on a settlement - but the deal satisfied neither the protesters nor the real authorities.)

The crisis may be almost over, or it may be just beginning - an unsatisfying way to end a column, but the Ukrainian people have had a very long unsatisfying history, and many of them won't want to crawl back to their slumber after seeing the first signs of a new, engaged political life in decades.

Fred Kaplan is the author of "The Insurgents" and the Edward R. Murrow press fellow at the Council on Foreign Relations.