DAVOS, Switzerland — President Donald Trump’s demands to take over Greenland, reiterated in an overnight barrage of social media posts, are transforming this week’s annual gathering of the global elite into an emergency diplomatic summit, as European leaders prepared to use the president’s arrival here Wednesday to de-escalate the spiraling crisis. Europe may not have a home-field advantage in Davos. The United States is seeking to dominate this year’s World Economic Forum by sending its largest and most senior delegation in history. Meetings with senior Trump officials are among the most sought-after engagements in town as European leaders, already reeling...