WASHINGTON (CNN) -

As Air Force One landed in Estonia's capital Wednesday, President Barack Obama's message to Vladimir Putin -- only 500 miles away in Moscow -- was clear: Stay put.

Obama's trip to the former Soviet state, ahead of this week's NATO summit in Wales, is meant to reassure nervous Eastern Europe that Putin's support for separatists in Ukraine doesn't mean he has a free pass for territorial gains elsewhere.

In a speech in Tallinn, Obama said the vision of a Europe dedicated to peace and freedom is "threatened by Russia's aggression against Ukraine," but said NATO will not allow that aggression to go unchecked.

"We will defend our NATO allies, and that means every ally," he said. "We will be here for Estonia. ... You lost your independence once before. With NATO, you will never lose it again."

Added to the schedule only last month, the stop in Estonia supplements the message coming from NATO leaders gathering in Cardiff, Wales, who are set to announce the positioning of troops and equipment closer to Russia in Eastern Europe.

In earlier comments alongside Estonia's President, Obama recalled the "deep ties" between the two nations as he announced plans for additional U.S. Air Force units to be based in Estonia as part of a bolstering of NATO forces in the region.

"One of our goals at the summit over the next several days is to once again project unity across NATO on behalf of Ukraine's efforts to maintain its sovereignty and territorial integrity," Obama said.

He added that Russia was "paying a heavy price for its actions," in part through Western sanctions imposed over Ukraine, and that NATO is poised to do more to help Ukraine defend itself.

He said more European NATO members need to spend a full 2% of their gross domestic product on defense to keep NATO strong.

The approaching NATO summit is an opportunity for these countries to pledge this, he said.

"Estonia does it. Every ally must do it," he said.

The NATO leaders also must confront the separate threat of militant Islamists making gains in Syria and Iraq, and the brutal beheading of a second American by ISIS, also known as ISIL or the "Islamic State."

Asked about his strategy on the extremist group, Obama said: "The bottom line is this: Our objective is clear, and that is to degrade and destroy ISIL so it's no longer a threat not just to Iraq, but also to the region and to the United States.

"In order for us to accomplish that, the first phase has been to make sure we've got an Iraqi government that's in place, and that we are blunting the momentum that ISIL was carrying out.

"And the airstrikes have done that. But now, what we need to do is make sure that we've got the regional strategy in place that can support an ongoing effort, not just in the air, but on the ground, to move that forward."

Analyst: NATO must adapt

The original reason for the summit was to determine how NATO's mission will proceed in Afghanistan when combat troops depart at the end of the year.

But the global unrest, while causing political strife for Obama in the United States, could provide a new purpose for the 65-year-old NATO alliance, which is suffering an "identity crisis," according to one analyst.

Putin's actions in Russia have "required NATO to really adapt and change fairly dramatically," said Heather Conley, who directs the Europe program at the Center for Strategic and International Studies in Washington.

"In some ways, NATO should thank Vladimir Putin, because it was really searching for its purpose," Conley said.

Warning Putin

NATO members that border Russia, watching the once unthinkable breach of Ukraine's borders, are looking to the military alliance to affirm its commitment to the North Atlantic Treaty's Article 5, which provides for collective defense of states under attack.

Ukraine isn't a NATO member, though leaders did invite the country's new President, Petro Poroshenko, to Wales this week. Other former Soviet states, like Estonia, Latvia, and Lithuania, joined NATO in the past decade, hoping to bolster ties to the West while increasing their own security frameworks.

As reassurance to those countries, NATO leaders plan to approve the creation of a "high-readiness" force that places new equipment and thousands of troops in Eastern Europe.

White House officials say the move is meant to be defensive rather than a provocation for Russia, though initial reaction from the Kremlin -- which called the move an "external threat" -- foreshadowed a potential escalation of the crisis.