According to a Kremlin statement, the two sides said they were ready to intensify military coordination in Syria.

Deterring Russian aggression on NATO's eastern flank will be a major topic of discussion among the NATO leaders over the course of the two-day summit. NATO recently decided to deploy four battalions to Poland, Estonia, Latvia and Lithuania to send a signal to the Russians.

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The United Kingdom's vote to leave the European Union will hang over the summit. "We're still in the shock phase," said Heather Conley, a senior vice president at the Center for Strategic and International Studies. The British decision is a major blow to more than 60 years of U.S. policy promoting European integration and trans-Atlantic cooperation, she said.

Obama has been a major proponent of this policy, and since the British decision to leave the European Union, he has tried to preach calm, saying that the departure, known as Brexit, does not pose a dire threat to the European Union. It's a theme he is likely to stress at the summit.

"This will be a very timely opportunity to discuss the aftermath of the Brexit vote and our continued, very strong support for the European Project, which has been at the center of so much security and prosperity for Europe and the United States and the world," said Ben Rhodes, a deputy national security adviser.

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This will be Obama's fifth NATO summit and probably his last trip to Europe as president. Administration officials said the array of issues being discussed will make it one of the most consequential in recent years. "By my count, there hasn’t been another inflection point like this for the alliance since the fall of the Berlin Wall and the dissolution of the Soviet Union in ’89 to ’91." Lute said. "So this is a bit of a historic point, and it’s a great point for the summit, especially as the president wraps up his tenure as the leader of the alliance."