U.S. Secretary of Defense Jim Mattis, right, and NATO Secretary General Jens Stoltenberg, second right, greet Greek Defense Minister Panos Kammenos, left, 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)
U.S. Secretary of Defense Jim Mattis, left, speaks with British Secretary of State for Defense Michael Fallon 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 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)
U.S. Secretary of Defense Jim Mattis waits for the start 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)
U.S. Secretary of Defense Jim Mattis, left, speaks with members of his delegation 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)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Tuesday, Feb. 14, 2017. NATO defense ministers will begin a two-day ministerial beginning on Wednesday. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Tuesday, Feb. 14, 2017. NATO defense ministers will begin a two-day ministerial beginning on Wednesday. (AP Photo/Virginia Mayo)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Tuesday, Feb. 14, 2017. NATO defense ministers will begin a two-day ministerial beginning on Wednesday. (AP Photo/Virginia Mayo)
German Defense Minister Ursula von der Leyen, right, and Lithuania's Presidend Dalia Grybauskaite speaks with a soldier during the NATO enhanced forward presence battalion welcome ceremony at the Rukla military base some 130 km (80 miles) west of the capital Vilnius, Lithuania, Tuesday, Feb. 7, 2017. The NATO enhanced forward presence battalion in Lithuania will be led by framework nation Germany. (AP Photo/Mindaugas Kulbis)
NATO Secretary General Jens Stoltenberg speaks during a media conference at NATO headquarters in Brussels on Monday, June 13, 2016. NATO's chief says the alliance will agree this week to send four multinational battalions to the Baltic states and Poland to boost their defenses against Russia. (AP Photo/Virginia Mayo)
NATO's supreme allied commander Europe U.S. Army General Curtis M. Scaparrotti addresses the media after a change of command ceremony at NATO military headquarters in Mons, southern Belgium on Wednesday May 4, 2016. U.S. Army General Curtis M. Scaparrotti was installed as NATO's 18th supreme allied commander Europe (SACEUR). The commander, by tradition an American general or admiral, is responsible for the overall direction and conduct of NATO's global military operations. (AP Photo/Geert Vanden Wijngaert)
In this Nov. 13, 2013, file pool photo, steel beams and a glass wall show on the exterior of the new NATO headquarters in Brussels. America’s substantial support for NATO, both in money and military aid, has long been a source of frustration for U.S. leaders, and questioned by some as a throwback to the Cold War era. (AP Photo/Virginia Mayo, Pool, File)
NATO's Supreme Allied Commander Europe, US Gen. Philip M. Breedlove waits for the start of a meeting of NATO's Military Committee at NATO headquarters in Brussels on Thursday, Jan. 21, 2016. In a one day meeting, NATO chiefs of defense will discuss issues of strategic importance to the Alliance ahead of the forthcoming Defense Ministers’ meeting in February. (AP Photo/Virginia Mayo)
U.S. Secretary of State John Kerry pauses before speaking during a media conference at NATO headquarters in Brussels on Wednesday, Dec. 2, 2015. U.S. Secretary of State John Kerry says NATO members stand ready to step up military efforts against the Islamic State. (AP Photo/Virginia Mayo)