One of NATO’s longest serving top officials, Secretary-General Jens Stoltenberg, stepped down on Tuesday, handing over the reins to former Dutch Prime Minister Mark Rutte as the military alliance confronts some of the biggest challenges in its history. The two men, who first sat together at NATO's table 14 years ago as the leaders of Norway and the Netherlands, greeted each other warmly outside the alliance's Brussels headquarters, before laying a wreath to fallen military personnel, surrounded by the flags of the 32 member countries.