The worry has intensified since Barack Obama came to power. Eager to prove themselves loyal allies, the new NATO members had cooperated with the United States—and then some—in the first decade of the 2000s. They had sent troops to the Iraq War. They had allowed the CIA to hold and question detainees on their territory. They had accepted a U.S. missile-defense system—even as the U.S. insisted that the system was intended to protect only against Iranian missiles (which didn’t threaten them) and not against Russian missiles (which did).

No Eastern European good deed went unpunished. Their cooperation with the CIA was leaked to the U.S. media, exposing them to accusations that they had violated European Union human-rights treaties. Their Iraq sacrifices counted little with an administration that wanted to exit that war on almost any terms. In the fall of 2009, the missile-defense system was canceled, ostensibly for technical reasons, but more likely (or so the Eastern Europeans believed) as part of the Obama administration’s hoped-for “reset” of relations with Russia.

Just as candidate Obama traveled to Germany in 2008 to dramatize how the Bush administration had alienated America’s traditional allies in Western Europe, so in 2012 Mitt Romney visited with former Polish President Lech Walesa to emphasize how badly the Obama administration had upset new allies in the East.

Now the long-dreaded crisis has arrived. Putin’s Russia has launched an escalating war against Ukraine. The number of dead is rising toward 3,000, including the 300 passengers aboard the Malaysia Airlines flight almost certainly brought down by a Russian-supplied missile. The justification for Putin’s aggression—Russia’s right to intervene on behalf of Russian-speaking minorities—could be applied against Estonia and Latvia as well as against Ukraine. And those small countries are far less capable of resisting Russia than Ukraine is. Pretty much all they have, in fact, is the guarantee of NATO’s Article 5: an attack on one member is an attack on all. Can that guarantee be relied upon? There has been no shortage of commentators arguing that it cannot.

True, the United States and Germany have stepped up their presence inside the Baltics and Poland since the annexation of Crimea. Elite special forces have been rotated into the three Baltic republics. Military equipment will be prepositioned in Poland. In March, NATO announced that it would hold exercises in western Ukraine at the end of September—exercises that look a lot like practice for a defensive war against a Russian invasion of an Eastern European country. Just this past week, NATO agreed to form a new "very high-readiness" brigade that could quickly deploy anywhere in Eastern Europe. Meanwhile, U.S.-led sanctions are exerting an ever-tightening grip on the Russian economy. Earlier this week, one gas-industry insider told The Financial Times that without access to U.S. technology, Russia’s hopes to develop a liquefied natural-gas industry would be squashed “like a bug.”