Two big questions about the future of multilateralism surfaced during last week’s United Nations General Assembly. How will the battle against climate change reshape international cooperation in the decades ahead? And will mounting competition between China and the United States render any cooperation impossible?The climate issue dominated the run-up to this General Assembly. Responding to dire warnings about global warming, Secretary-General Antonio Guterres insisted that it must be front and center in New York. He oversaw a masterful bout of diplomatic choreography, as the U.N. welcomed young, superstar activist Greta Thunberg to address a special Climate Action Summit, pushing leaders to make concrete pledges to reduce carbon emissions. ...