In August 2018, high in the sky over northeastern China, the Xingkong-2 hypersonic cruise missile raced at Mach 6, or six times the speed of sound, bobbing and weaving through the atmosphere. This byproduct of the rapidly developing field of hypersonic weapons is merely one of at least three such missiles purportedly in China’s arsenal. Meanwhile, Russia and the United States are in the thick of developing hypersonic weapons of their own. At the heart of this technology, which is now undefendable according to sources in the U.S. military, lies a wickedly simple engine, known as a scramjet. “Air-breathing” scramjets enable these weapons to expertly maneuver at a hypersonic pace—speeds of Mach 5 or greater—making them extremely difficult to track and neutralize. Their potential has led to what is essentially a modern “space race,” in which China, Russia, and the United States are jockeying to deploy accurate, highly functional hypersonic weapons.