To understand what Vladimir Putin is really up to in Ukraine—why he is willing to take the kinds of risks that produced the destruction of a civilian airliner, and why the U.S. and its allies should see his power play as an effort to alter not just the arc of Ukraine but all of Europe—it's necessary to look at the tale of two countries.

The first is Poland, a country of 38 million. After the end of the Cold War, this former Warsaw Pact nation turned westward. It almost immediately sought membership in the European Union and joined the North Atlantic Treaty Organization in 1999. After modernizing its economy, it officially became part of the EU in 2004.

Next door to Poland lies Ukraine, a country of 44 million. After the end of the Cold War, this former Soviet satellite didn't turn west but rather stayed focused on its traditional relationship with Russia to the east.

What has happened to these two neighbors in the quarter-century since the Berlin Wall fell? In a nutshell, they have moved in opposite directions.

Poland, the country that integrated itself into the Western economy, has grown almost twice as fast as Ukraine. Last year, its growth rate was three times larger. Though it's the slightly smaller of the two neighbors, Poland now has a gross domestic product more than twice the size of Ukraine's. It has only half the share of its population living under the poverty line as does Ukraine.

This is the contrast that must scare Mr. Putin. It also is the one that set off alarm bells when Ukraine, emulating neighboring Poland, began to pivot westward earlier this year. To allow that turn to happen, in the most important of Russian satellites, would have been the end of any near-term dreams of rebuilding a Russian empire.

In short, the goal of re-creating a Russian sphere of influence was colliding head-on with the spread of a Westernized, EU model for Europe, which was seeping toward Russia's doorstep. Mr. Putin faced a historic choice: swim with the tide or try to turn it. He chose the latter.

"I think [Russia's] goal is a weak and divided Ukraine, and a bigger goal is a weak and divided Europe—a weak and divided EU," says Robert Hormats, under secretary of state in the first Obama term. Moreover, to the extent a country such as Poland prospers, he adds, "it creates a very, very stark contrast to the troubled economic prospects in Russia" itself.

Mr. Putin had to move quickly to reverse those trends, for he is at a moment of relative but passing strength. Today, Western Europe's dependence on Russian natural gas gives him some economic leverage. As Europe has gobbled up more of Mr. Putin's gas, EU trade with Russia has tripled in value over the last decade.

This Russian economic advantage doesn't figure to last; eventually, Europe will wean itself away from its dependence on Russian carbon fuels. But for now, Mr. Putin must have calculated, he could make his play in Ukraine and face a muted Western response.

And if that was his calculation, he was mostly correct. Business interests, not just in Europe but in the U.S., have resisted toughening economic sanctions. Perhaps the downing of an airliner has changed that; we'll learn more at a meeting of EU leaders Tuesday.

This also explains why Poland looks with alarm at Russian bullying of Ukraine, and at the Western response so far. Poland knows from history that it is vulnerable to being yanked back toward the east, so it seeks more help from the Western club to which it now belongs.

"The crisis in Ukraine could have been prevented," Polish Foreign Minister Radoslaw Sikorski said in an interview with German journalists published over the weekend. "Europe has done too little to influence Russia's behavior in the different stages of the conflict. When a Russian trade boycott against Ukraine was imposed last year to punish it for its European course, I pleaded with my colleagues to take action." If the West had moved then, he added, today's "escalation" probably would have been avoided.

If Poland is indeed the success story, it's a particularly troubling note for President Barack Obama that a Polish magazine last month quoted Mr. Sikorski as saying, in a leaked tape of a private conversation, that Poland's defense ties to the U.S. were "worthless."

In his weekend interview he was more diplomatic but still argued for more Western defense help to avoid Ukraine's fate. "The reality [is] that there are large [Western] military bases in countries that are safe. And there is a hesitation to build these bases in states that feel threatened."