President Trump is considering a significant change to U.S. participation in NATO, according to three current and former senior U.S. officials and one congressional official. Trump has discussed with aides the possibility of adjusting America’s NATO engagement to favor members that allocate a set percentage of their GDP to defense, the officials said. As part of this potential shift, the U.S. might not come to the defense of a NATO ally under attack if the country fails to meet the defense spending threshold, the officials said. Such a move would mark a major departure from NATO’s Article 5, which states...