Bangladesh, July 8 -- More than seven decades after its creation, the North Atlantic Treaty Organization (NATO) remains the worlds most powerful military alliance. Built in the aftermath of the Second World War to deter Soviet expansion and safeguard the democratic West, NATO has endured the Cold War, adapted to new security threats, and expanded its membership across Europe. Yet despite its unmatched military capabilities, the alliance now faces a deeper and more complex challenge than any external rival. Its greatest test lies not in confronting Russia, China, or regional instability, but in answering a more fundamental question: What exactly is NATO defending in the twenty-first century?

Military alliances derive their strength not on...