In many capitals, Mr. Trump’s formal and off-the-cuff foreign policy proposals — his threat to pull out of NATO; his musings about removing the United States’ nuclear umbrella over Japan and South Korea; his pledge to slap huge trade tariffs on China — are regarded with a mix of alarm and confusion. Asked on Thursday if Beijing was concerned about the prospect of a Trump presidency, the Chinese Foreign Ministry spokesman, Hong Lei, replied, “We hope the U.S. people from all walks of life would view bilateral relations from a reasonable and objective perspective.”

Stefano Stefanini, a former representative of Italy to NATO and former diplomatic adviser to the Italian president, put it this way: “There is no Donald Trump contingency plan.”…

Officials do not see Mr. Trump’s rise as merely an American version of the anti-immigration and isolationist parties that have picked up support across Europe. They are finding signs of tangible political change in statements by Democratic leaders, as well.

Already, Mr. Trump’s assertive positions about American interests have led some officials to look again at President Obama’s recent critique of European and Persian Gulf allies as “free riders.” They have also helped shed light abroad on the domestic political forces at play around Hillary Clinton’s decision to renounce her support for a new Asian trade deal.