President Obama arrives in Estonia Wednesday on a high-stakes mission ahead of a NATO summit to reassure worried allies in Eastern Europe that the U.S. will defend them.

The visit comes at a defining time for NATO, whose leaders must reinforce the message that they are committed to defending all of its members to preserve the alliance's credibility — a key concern for Estonia, where ethnic Russians make up a quarter of the population and Moscow has already hinted that their welfare is Russia's business.

“Everyone in Washington, Berlin, London and Paris should understand that if you lose even a tiny bit of NATO territory to an aggressor, it’s the end of the alliance and the security architecture of the world as we know it today,” Estonian Defense Minister Sven Mikser told the Financial Times in an interview.

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

"You have in Estonia a large Russian population, and therefore part of the message that the president will be sending is, we stand with you ... Russia, don’t even think about messing around in Estonia or in any of the Baltic areas in the same way that you have been messing around in Ukraine."

Among the agenda items for the NATO summit Thursday and Friday in Wales is creation of a rapid-response force to respond quickly to threats and show potential aggressors that the alliance is serious about defending its eastern members with a more visible presence — a key concern of not just the Baltic states of Estonia, Latvia and Lithuania, but also of Poland.

"We do believe that there is a very strong deterrent effect of having such a response force at very high alert so that any potential aggressor knows that if they even started to think about attacking a NATO ally they would meet not only national troops from that specific NATO ally but they would meet NATO troops," NATO Secretary General Anders Fogh Rasmussen said Monday at a news conference, noting that Moscow's aggressive behavior means "we must face the reality that Russia does not consider NATO a partner."

The proposed rapid-reaction force has already ratcheted up tensions with Russia, which in response declared NATO a major external threat on Tuesday, making it more difficult to find a way to peacefully keep the government of Russian President Vladimir Putin from believing he has a free hand to dictate Ukraine's future.

Obama and the other NATO leaders also must discuss what to do about Ukraine, a difficult question that may affect whether the alliance will have to defend its boundaries with force.

Western leaders, including Obama, have refused to recognize Russian and Russian-backed separatists claims to Crimea and other Ukrainian territories but have insisted that military force will not be used to roll back their gains, and the U.S. so far has declined to provide lethal military aid to Kiev's forces.

U.S. officials estimate thousands of Russian troops are inside Ukraine and more than 10,000 others are massed along that country's border ready to move at a "literally on a moment's notice," according to Pentagon spokesman Rear Adm. John Kirby. Though the Obama administration insists that Western sanctions have hurt Moscow, they have not deterred Putin from fueling the conflict.