The US Air Force’s scramjet-powered Hypersonic Attack Cruise Missile (HACM) has finally reached the flight-test phase, with at least one release carried out over a test range in Australia. The programme has suffered significant delays, prompting the Air Force to scale back its overall testing plans in an attempt to keep it on schedule. Its stated goal is to decide whether to begin rapid acquisition of the missile by 2027.HACM has been developed jointly with the Royal Australian Air Force, partly under the broader US-Australian Southern Cross Integrated Flight Research Experiment (SCIFiRE), while also drawing on the Defense Advanced Research Projects Agency’s (DARPA) earlier Hypersonic Air-breathing Weapon Concept (HAWC) project. Raytheon, now RTX, working alongside Northrop Grumman, was selected to develop the missile in 2022. The weapon has reportedly since been designated AHM-4A under a new branch of the US military’s joint nomenclature system.Two-stage, air-breathing hypersonic designs of this kind have been tested before, including Boeing’s scramjet-powered X-51 Waverider and the HyFly 2 vehicle, both of which used a waverider configuration to skip across their own shockwaves for additional lift.India’s own hypersonic ambitions received a significant boost with the successful test of its Hypersonic Technology Demonstrator Vehicle, which sustained speeds beyond Mach 6 and marked a breakthrough in scramjet propulsion. The demonstrator lays the groundwork for future hypersonic cruise missiles capable of penetrating advanced air defences, placing India alongside the US, Russia and China in a domain where speed itself is redefining deterrence by shrinking reaction windows to mere seconds.Hypersonic weapons, travelling above Mach 5, or well over 6,000 km an hour, broadly fall into two categories: boost-glide vehicles launched by rockets that manoeuvre mid-flight, and scramjet-powered cruise missiles capable of sustained hypersonic flight. Unlike ballistic missiles, which follow predictable arcs, these weapons can change course mid-flight and fly low enough to exploit a radar horizon that can shrink to as little as 15 km, making interception with current missile defence systems extraordinarily difficult.Building them is just as hard as stopping them. The extreme thermal loads at Mach 5 and above demand advanced cooling systems, while scramjet combustion stability is notoriously difficult to sustain, often compared to keeping a candle lit in a hurricane. The ionised air sheath surrounding the vehicle at such speeds can also cut off communication entirely, the same phenomenon spacecraft experience during re-entry. DRDO has set its own benchmark here, successfully testing an actively cooled, full-scale scramjet combustor for 1,200 seconds.India’s broader hypersonic portfolio includes the HSTDV, the planned BrahMos-II cruise missile, and concepts such as the Dhvani glide vehicle, the Long Range Anti-Ship missile and the ET-LDHCM extended trajectory cruise missile. Together, they signal a determined push towards indigenous hypersonic capability. Globally, the US, China, Russia, France and Australia continue to invest heavily in this space, underlining that hypersonic warfare is no longer a distant prospect but an unfolding reality set to reshape future battlefields.