Hypersonic weapons have been under development for a couple of decades, and are still up to a decade away from deployment; but they may complicate the defensive equation to a very great extent. Here’s a discussion about them from the RAND Corporation that shows how they may change things. It’s worth watching.
The USA, Russia and China are the furthest advanced in developing hypersonic weapons. Several other first-world nations (Britain, France, Australia, and a few others) are several years behind, but working hard to catch up (because they don’t want to be left out of the new technology). Some third-tier powers, such as South Africa, have preliminary programs looking at the field, trying to work out what they need to do to participate. (The likely answer will be to try to steal and/or build on the research already done by major powers – that’s been the case for many years, and it works, so it’ll likely continue. That’s how China developed much of its present military technology.)