By any measure, António Guterres of Portugal is an excellent choice to replace Ban Ki-moon of South Korea as the next United Nations secretary general. He has experience, energy and diplomatic finesse, all of which he’ll need to lead the United Nations as it confronts regional wars, rising tensions between Russia and the West, China’s aggressive posture in Asia and the refugee crisis in the Middle East and Europe.

Against these challenges, the secretary general post has only limited power, and its diplomatic influence is even more attenuated with stateless terrorist groups and insurgencies that cross international borders. A good part of Mr. Guterres’s work will be to figure out how the United Nations, a 193-member body, can navigate a world in which terrorism and war are melded and now are driven by multiple forces.

After leading Portugal as prime minister, Mr. Guterres served as the United Nations high commissioner for refugees for a decade until 2015, dealing with the displacement of millions fleeing wars in Syria, Afghanistan, Iraq and elsewhere by providing food and shelter and finding them refuge in other countries. Mr. Guterres was effective at pressing Western nations to do more to help and at hammering out agreements in difficult circumstances. As the refugee crisis has worsened, it has generated a nationalistic backlash in Europe and the United States. Mr. Guterres’s understanding of the problem and his passionate advocacy for just and compassionate solutions could persuade governments to keep accepting refugees, rather than shut them out.