World leaders were gathering in the Netherlands on Tuesday for the start of a historic two-day NATO summit that could unite the world’s biggest security organization around a new defense spending pledge or widen divisions among the 32 allies. The allies are expected to endorse a goal of spending 5% of their gross domestic product on their security, to be able to fulfil the alliance’s plans for defending against outside attack. U.S. President Donald Trump’s first appearance at NATO since returning to the White House was supposed to center on how the U.S. secured the historic military spending pledge from others in the security alliance — effectively bending it to its will.