Bangladesh, June 25 -- As the NATO summit in The Hague kicks off amid growing geopolitical tension, US President Donald Trump has once again stirred controversy-this time by casting doubt on one of the alliances foundational principles: Article 5, the mutual defense clause. When asked by a reporter during his flight aboard Air Force One whether he remained committed to the articles promise of collective defense, Trump gave a characteristically evasive and provocative reply: “Depends on your definition.”

“There are numerous definitions of Article 5. You know that, right?” Trump said. “But Im committed to being their friend. You know, Ive become friends with many of those leaders.”

His comments have tri...