At NATO headquarters, where he will visit Thursday, aides have prepped Secretary-General Jens Stoltenberg for the possibility that the president could try to pull off a stunt such as passing around invoices to member countries who have not met the alliance's financial guidelines, according to a person with knowledge of the planning.
USA first lady Melania Trump cheered up children in a Belgian hospital with Dr. Seuss books and crepe paper flowers while her husband met with European Union leaders.
In total, Trump will spend about 24 hours in Brussels, a city where he said making a home would be "like living in a hellhole" because of Muslim immigration and terror threats.
As the Turkish authorities suppress all criticism of its leaders by labeling them terrorists, North Atlantic Treaty Organisation member states should be extremely concerned about the extent to which Turkey can be a reliable ally in countering the real threats of terrorism and violent attacks or cooperating successfully in upholding global security.
Stoltenberg dismissed the notion that NATO's joining the coalition was mere symbolism, saying it is a "practical platform" to coordinate efforts.
But as Russian Federation continues to maintain its aggressive posture in Eastern Europe and the Baltic on the heels of its annexation of Crimea and amid its continued troop presence in Eastern Ukraine, NATO members remain on edge and on guard. That was the pattern during the Cold War, too.
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Summoning from the Europeans a greater collective effort for their common defense is all the more hard today because NATO's European members are divided in important ways. The leader of another member, Canadian Prime Minister Justin Trudeau, declined to answer directly Thursday whether he was reconsidering his country's intelligence agreement with the United States.
However, EU officials believe he has come to a greater appreciation since taking office of the value of European integration to USA interests. Trump will discover on his trip, if he has not already, that all his foreign counterparts will want something from him.
"At that time the questions were on Brexit", and on the survival of the EU, a senior European official said on condition of anonymity.
American global leadership must come, as it has for more than a century, from the Oval Office.
Mr Trump also met Italy's president and prime minister while in Rome.
Trump's first meeting with senior European Union leaders kicked off a flurry of diplomatic activity in Brussels.
Michael Mandelbaum is professor emeritus of American foreign policy at the Johns Hopkins University School of Advanced International Studies. - Ed.