Gregory Korte

USA TODAY

CORRECTIONS AND CLARIFICATIONS: Previous versions of this story misstated the number of NATO countries bordering Russia. In addition to the three Baltic States —Estonia, Latvia and Lithuania — Norway shares a 122-mile border with Russia above the Arctic Circle, and Poland neighbors the Kaliningrad Oblast enclave.

WASHINGTON — President Obama leaves Tuesday for a four-day European trip meant to reassure NATO allies and send a stern message to Russia over the crisis in Ukraine.

The first stop: The Baltic nation of Estonia, one of five NATO countries that directly borders Russia and where a quarter of the population are ethnic Russians. Then on Thursday, Obama meets with other NATO leaders in Wales, with Ukraine tops on the agenda.

"It is clearly not accidental that the president has decided to stop in Estonia on the way to the NATO Summit," said Charles Kupchan, the senior director for European Affairs at the National Security Council. "The two stops are essentially part of the same effort to send a message to the Russians that their behavior is unacceptable."

The catchword of the European trip will be "reassurance."

Article 5 of the North Atlantic Treaty holds that an attack on one member is an attack on all members. Obama said last week that the NATO Summit needs to "refocus attention" on those assurances.

"Part of the reason I'll be going to Estonia is to let the Estonians know that we mean what we say with respect to our treaty obligations," Obama said last week, quickly noting that the United States has no such obligations with Ukraine, which is not a member of NATO. "We do, however, stand shoulder to shoulder with them," he said.

While in the Estonian capital of Tallinn, Obama will meet with the heads of all three Baltic nations — Estonia, Latvia and Lithuania — and tour an airport hangar where he will greet U.S. and Estonian troops. One focus of the talks will be to increase the readiness of U.S. forces in Eastern Europe through increased troop rotations and joint training exercises.

On Thursday, Obama flies to Newport, Wales, host of the 2014 NATO Summit. Though Ukraine is not a NATO member, the alliance has established a NATO-Ukraine Commission to give the former Soviet republic a seat with western leaders. Ukrainian President Petro Poroshenko will attend meetings of the commission in Wales.

Those talks come the same week that the European Union has given Russia a one-week ultimatum to withdraw from Ukraine or face additional economic sanctions. Twenty-two of NATO's 28 members are also members of the EU.

Though Afghanistan, Iraq and Syria will get significant attention at the summit, Ukraine is helping to drive a more fundamental rethinking about the role of the North Atlantic alliance 65 years after it was founded to confront Soviet expansionism.

"I think one of the things that we've learned from the situation in Ukraine is that oftentimes in this new world that we live in, NATO or individual countries may be facing not armored columns coming across their border, which you can usually see in advance, but guys coming across in masks, you don't know who they are," Kupchan said.

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