British military soldiers walk past a scale model of a F35 jet forming part of an exhibit on the grounds of the Celtic Manor Resort prior to a NATO summit in Newport, Wales on Wednesday, Sept. 3, 2014. NATO heads of state meet for a two-day summit beginning on Thursday. (AP Photo/Jon Super)
NATO soldiers of U.S. and Lithuania, right, take part in a military exercise 'Saber Strike 2018' at the Training Range in Pabrade some 60km (38 miles) north of the capital Vilnius, Lithuania, Monday, June 11, 2018. A major U.S.-led military exercise with 18,000 soldiers from 19 primarily NATO countries is taking place in the alliance's eastern flank involving Poland and the three Baltic states of Estonia, Latvia, Lithuania. (AP Photo/Mindaugas Kulbis)
Defense Secretary James N. Mattis will be pressuring NATO members to do more on and off the battlefield. The U.S. pressure for more defense spending and support from key NATO allies is having an impact, analysts say. (Associated Press Photographs)
Montenegrin guards of honor stand between NATO, left, and Montenegro flags during ceremony to mark Montenegro's accession to NATO, in Podgorica, Montenegro, Wednesday, June 7, 2017. Despite opposition from Russia, Montenegro joined NATO on Monday after completing its membership formalities in Washington. (AP Photo/Risto Bozovic)
NATO Secretary General Jens Stoltenberg attends a joint press conference with Ukrainian President, Petro Poroshenko in Kiev, Ukraine, Monday, July 10, 2017. Speaking Monday with NATO Secretary General Jens Stoltenberg in Kiev, Ukraine President Poroshenko said Ukraine would not be applying for a NATO membership "immediately" but would instead "build a genuine program of reforms" to meet NATO requirements for membership in the future. (AP Photo/Efrem Lukatsky)
U.S. Secretary for Defense Jim Mattis, center, waits for the start of a bilateral meeting on the sidelines of a NATO defense ministers meeting in Brussels on Thursday, June 29, 2017. NATO defense ministers meet Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo, Pool)
British Defense Minister Michael Fallon, second left, speaks with German Defense Minister Ursuala von der Leyen, left, while NATO Secretary General Jens Stoltenberg, right, speaks with U.S. Secretary for Defense Jim Mattis during a meeting of NATO defense ministers at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers met Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo)
U.S. Secretary for Defense Jim Mattis, center, arrives for a meeting of NATO defense ministers at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers met Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo)
U.S. Secretary for Defense Jim Mattis, center, arrives for a meeting of NATO defense ministers at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers met Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo)
U.S. Secretary for Defense Jim Mattis, left, speaks with NATO Secretary General Jens Stoltenberg during a meeting of NATO defense ministers at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers met Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo)
U.S. Secretary for Defense Jim Mattis, center, arrives for a meeting of NATO defense ministers at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers met Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo)
U.S. Secretary for Defense Jim Mattis, left, shakes hands with NATO Secretary General Jens Stoltenberg prior to a meeting at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers meet Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo, Pool)
U.S. Secretary for Defense Jim Mattis, center left, takes his seat along with NATO Secretary General Jens Stoltenberg, right, prior to a meeting at NATO headquarters in Brussels on Thursday, June 29, 2017. NATO defense ministers meet Thursday to discuss, among other issues, the situation in Afghanistan and defense spending. (AP Photo/Virginia Mayo, Pool)