ICBM reentry vehicles are hypersonic as well, but are (mostly) ballistic. The modern usage of the term "hypersonic missile" refers to maneuverable hypersonic mi

ICBM reentry vehicles are hypersonic as well, but are (mostly) ballistic. The modern usage of the term "hypersonic missile" refers to maneuverable hypersonic missiles that can more effectively evade missile defense systems Hypersonics are being made primarily to ensure MAD. The US is building a number of ballistic missile defense (BMD) systems, including Aegis BMD, THAAD, Patriot (to an extent), and GBI. There's enough of these to defend against a handful of missiles launched from North Korea or Iran, but not against all of Russia's fleet (assuming significant numbers of them still work of course). China has a significantly smaller nuclear arsenal, so they may be worried that the US could eventually have enough BMD to render their threat of MAD non-credible. Plus, they also want conventional hypersonics to better attack carrier strike groups I'm not totally sure why the US wants to build nuclear hypersonics, since they have enough nukes in high enough readiness that nobody's going to be able to eliminate the threat of US MAD anytime soon. I think it's being done for simple research purposes so they have an idea what their enemies' hypersonics are like, and for prestige reasons