AS THE shock waves from Russia’s invasion of Ukraine spread across Europe, the first country to tremble is neighboring Moldova, a small country that, since 1992, has been challenged by a separatist province propped up by Russian troops. Transnistria, a lawless zone that survives on profits from smuggling, has been relatively quiet in recent years. But Moldova, like Ukraine, has been seeking economic integration with the European Union while turning aside rival offers — and resisting mounting pressure — from Vladi­mir Putin.

Will Mr. Putin move to make the “frozen conflict” in Transnistria hot if Moldova presses ahead? Moldovan Prime Minister Iurie Leanca can only “hope it won’t lead to further destabilization,” he told us Monday. Perhaps fortuitously, he was in Washington as the Obama administration grappled with how to respond to the Russian takeover of Crimea. President Obama joined the prime minister’s meeting with Vice President Biden, which was appropriate: Part of the U.S. response must be to take steps to reassure Moldova and a host of other nations threatened by Russia’s act of aggression.

Moscow’s blatant violation of the European borders it pledged to respect following the Cold War raises especially pressing concerns for countries that used to be Soviet republics, such as Moldova and Georgia, which also has lost parts of its territory to separatist regimes backed by Russian troops. Three others — Estonia, Latvia and Lithuania — are members of NATO and have substantial Russian minority populations that the Kremlin has sought to manipulate in the past.

Whether Mr. Putin seeks to replicate his Ukrainian adventure in these nations will depend in part on how strongly the West responds to the Crimean invasion but also on whether it visibly strengthens other borders against Russia. That will mean moving forward on a broad menu of measures that have been inconclusively discussed in and outside NATO for years.

An obvious first step is to dedicate greater NATO resources to training, exercises and defense planning in members along the border with Russia, starting with the Baltic states and extending to Romania and other former Warsaw Pact countries. These governments’ requests for more NATO deployments of troops, missile defenses and other facilities have often been deferred on the grounds that no serious threat existed or that action would provoke Moscow. That logic no longer holds.

NATO should also reinvigorate its glacial move to expand to nations in southeastern Europe. Pending applications from Monte­negro and Macedonia could be put on the agenda for a summit this year, along with a long-delayed membership action plan for Georgia. Mr. Putin must get the message that his aggression will not retard the integration into Western security structures of new European democracies.

Moldova is not seeking NATO membership. But Mr. Leanca says it is vital that its E.U. association agreement move forward and that E.U. leaders take the step of providing what he called a “European perspective” — a path for Moldova eventually to obtain E.U. membership.

“The lesson of Ukraine is that our countries need a clear European perspective,” the prime minister said. “If we had this perspective, maybe we would not have these problems.” He’s right.