Why is Vladimir Putin tossing away his $50 billion Olympic effort to polish Russia's international image by intervening militarily in Ukraine?

It helps to understand the mindset of this balding former KGB colonel who clawed his way up under Boris Yeltsin to become president 14 years ago. (I actually crossed paths with him when he was an unknown aide to the corrupt mayor of St. Petersburg in the early 1990s).

This is a guy who has called the collapse of the Soviet Union "the greatest geopolitical catastrophe of the (20th) century." Like many Russians, Putin is deeply suspicious of what's called nazapad — the West — and fears being surrounded by enemies. Devastating invasions by Napoleon in 1812 and Hitler in 1941, which killed tens of millions, are huge parts of this Russian mindset that Americans may not fully appreciate. The fact that three former Soviet republics (Latvia, Lithuania and Estonia) are now members of NATO also fuels this paranoia.

History and geography loom large in ways that American policymakers and the American public may not grasp. And when it comes to Ukraine — Europe's largest country — Putin sees unrest in his backyard, with millions of people, largely in the western half of that country, yearning for closer ties with the West. This makes Putin frightened.

It's this paranoia that continues to make Russia — nearly a quarter-century after the Soviet collapse — so dangerous. The geopolitical fallout extends far beyond Crimea; Russia is now likely to be even less cooperative on critical matters like Syria, its role in the P5+1 nuclear talks with Iran, and more. It controls natural gas supplies to much of Western Europe, a nasty form of leverage that Putin would almost certainly use if needed. If the Cold War ended with the fall of the Berlin Wall in 1989 and collapse of the U.S.S.R. itself two years later, then it's fair to say that the post-Cold War era is now over; what's next is anyone's guess.

So what can President Obama do about it?

Not much.

The fact is that Obama, like numerous presidents before him, won't — and really, can't — do much to stop Putin from acting in what he sees as his own backyard. The Soviet Union, and now Russia, has a long history of using brute force when it feels the need — and the United States has usually allowed it to happen.

In October 1956, Dwight Eisenhower ignored the pleas of Hungarian freedom fighters who were rebelling against Soviet rule. Ike, who had fanned the flames by saying he supported the "liberation" of "captive peoples" in Eastern Europe, did nothing when the chips were down, fearing a nuclear exchange with Moscow. Soviet tanks rolled.

A dozen years later, a similar revolt erupted in Soviet-controlled Czechoslovakia. The rebellion, dubbed the "Prague Spring," prompted a Moscow-led invasion. Lyndon Johnson's response? A stiff statement condemning the move as a clear violation of the United Nations Charter.

That was it.

One exception to this American docility: Ronald Reagan's response to the Soviet invasion of Afghanistan. The 1979 attack prompted Jimmy Carter to cut off sales of wheat and to order a boycott of the Moscow Olympics. And when Reagan became president, he took it to the Soviets, training the mujahideen and arming them with Stinger missiles. His moves to roll back Soviet influence, as opposed to merely containing it, deserve much credit for forcing Moscow's retreat.

But fast forward 20 years, and you'll find President George W. Bush, who in 2008 reverted to the Eisenhower/LBJ/Carter style of talking tough and then backing down. Bush had encouraged Georgia, a former Soviet republic, to apply for NATO membership and accept American arms and training. But Vladimir Putin had no intention of allowing America too close to its "near abroad" (as the Kremlin calls its border regions), and the Russian invasion was on. What did Bush do? "I was very firm with Vladimir Putin," the president said. "Hopefully this will get resolved peacefully." But Bush failed to support the Georgians. Moscow grabbed big chunks of its territory.

Other than the Reagan exception, the record is clear. When it comes to Russia, American presidents often talk tough but do nothing. Expect the same from President Obama even as Russian troops head toward Crimea.

Ukraine, after all, is even closer (in terms of both history and geography) to Russia than any of the above examples. To think Obama or any president could stop the Russian bear in its own backyard is naïve.

Russia traditionally feels secure only when its neighbors do not; today they do not.

Do not expect Putin to back down. And do not expect President Obama to stand up to him. That would be a historical aberration.