What are ATACMS, US missiles that Ukraine used to target Russia? What happens next?

Russia has said that Ukraine has used the Army Tactical Missile System (ATACMS), provided by the US, to hit its Bryansk region. The missiles, which have a range of 300 kilometres, fly high into the atmosphere before gaining speed and smashing into their targets – thus foiling air defence systems. Though Moscow has responded by lowering the threshold for using nuclear weapons, experts say ATACMS are not a game-changer