António Guterres presides over his final United Nations General Assembly in New York this September, closing a decade of leadership during which the world's foremost diplomatic institution found itself unable to halt the wars in Ukraine and the Middle East. His tenure illuminates a tension as old as multilateralism itself: the gap between the authority to speak and the power to act. As he steps away, the assembly convenes not in triumph but in stalemate, a reminder that the architecture of global cooperation was built for a world that geopolitics has not yet agreed to inhabit.