British Prime Minister Theresa May arrives for the NATO summit in Brussels on Thursday, May 25, 2017. US President Donald Trump and other NATO heads of state and government on Thursday will inaugurate the new headquarters as well as participating in an official working dinner. (AP Photo/Geert Vanden Wijngaert)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Wednesday, May 24, 2017. NATO will host a Heads of State summit in Brussels on Thursday. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Wednesday, May 24, 2017. NATO will host a Heads of State summit in Brussels on Thursday. (AP Photo/Virginia Mayo)
Flags flap in the wind in front of NATO headquarters in Brussels on Friday, March 31, 2017. U.S. Secretary of State Rex Tillerson meets with his NATO counterparts in Brussels for the first time looking to persuade reluctant allies to increase defense spending and do more to combat terrorism. (AP Photo/Virginia Mayo)
Canadian Foreign Minister Chrystia Freeland speaks with the media as she arrives for a meeting of NATO foreign ministers at NATO headquarters in Brussels on Friday, March 31, 2017. U.S. Secretary of State Rex Tillerson meets with his NATO counterparts in Brussels for the first time looking to persuade reluctant allies to increase defense spending and do more to combat terrorism. (AP Photo/Virginia Mayo, Pool)
U.S. Secretary of State Rex Tillerson, center left, and NATO Secretary General Jens Stoltenberg, center right, shake hands as they wait for the start of a meeting of the North Atlantic Council at NATO headquarters in Brussels on Friday, March 31, 2017. U.S. Secretary of State Rex Tillerson meets with his NATO counterparts in Brussels for the first time looking to persuade reluctant allies to increase defense spending and do more to combat terrorism. (AP Photo/Virginia Mayo)
U.S. Secretary of State Rex Tillerson, left, shakes hands with NATO Secretary General Jens Stoltenberg during a meeting of the North Atlantic Council at NATO headquarters in Brussels on Friday, March 31, 2017. U.S. Secretary of State Rex Tillerson meets with his NATO counterparts in Brussels for the first time looking to persuade reluctant allies to increase defense spending and do more to combat terrorism. (AP Photo/Virginia Mayo)
U.S. Secretary of State Rex Tillerson, left, shakes hands with NATO Secretary General Jens Stoltenberg during a meeting of the North Atlantic Council at NATO headquarters in Brussels on Friday, March 31, 2017. U.S. Secretary of State Rex Tillerson meets with his NATO counterparts in Brussels for the first time looking to persuade reluctant allies to increase defense spending and do more to combat terrorism. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg speaks during a media conference after a meeting of the NATO-Russia council at NATO headquarters in Brussels on Thursday, March 30, 2017. Ambassadors from NATO and Russia met for the first time this year in a fresh attempt to resolve some of their differences. (AP Photo/Virginia Mayo)
U.S. Secretary of Defense Jim Mattis speaks during a media conference at NATO headquarters in Brussels on Thursday, Feb. 16, 2017. U.S. Defense Secretary Jim Mattis on has told NATO ministers that the alliance is "a fundamental bedrock for the United States" while at the same time demanding an increased financial commitment from the 27 other alliance members. (AP Photo/Virginia Mayo)
U.S. Secretary of Defense Jim Mattis speaks during a media conference at NATO headquarters in Brussels on Thursday, Feb. 16, 2017. U.S. Defense Secretary Jim Mattis on has told NATO ministers that the alliance is "a fundamental bedrock for the United States" while at the same time demanding an increased financial commitment from the 27 other alliance members. (AP Photo/Virginia Mayo)
U.S. Secretary of Defense Jim Mattis, center, speaks with Canadian Defense Minister Harjit Singh Sajjan, right, during a meeting of the NATO-Georgia Council at NATO headquarters in Brussels on Thursday, Feb. 16, 2017. U.S. Defense Secretary Jim Mattis on Wednesday told NATO ministers that the alliance is "a fundamental bedrock for the United States" while at the same time demanding an increased financial commitment from the 27 other alliance members. (AP Photo/Virginia Mayo)
U.S. Secretary of Defense Jim Mattis, second right, speaks with German Defense Minister Ursula von der Leyen prior to a meeting at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo, Pool)
German Defense Minister Ursula von der Leyen, center, motions toward U.S. Secretary of Defense Jim Mattis, left, prior to a group photo of the Enhanced Forward Presence members at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. NATO's Enhanced Forward Presence is an allied, forward deployed defense and deterrence posture in Eastern Europe to protect and reassure NATO's Eastern member states. (AP Photo/Virginia Mayo, Pool)
U.S. Secretary of Defense Jim Mattis, left, speaks with Turkish Defense Minister Fikri Isik during a meeting at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo, Pool)
NATO defense ministers pose for photographers during a group photo at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo)
U.S. Secretary of Defense Jim Mattis, right, speaks with British Secretary of State for Defense Michael Fallon during a meeting of the North Atlantic Council at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg, front left, and U.S. Secretary of Defense Jim Mattis, front left, stand with other NATO defense ministers during a group photo at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg, right, and U.S. Secretary of Defense Jim Mattis prepare to address a media conference at NATO headquarters in Brussels on Wednesday, Feb. 15, 2017. For U.S. Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist. He is expected to tell allies the U.S. is committed to NATO and is also hoping to secure bigger defense spending commitments. (AP Photo/Virginia Mayo)