The US and Russia did promise to defend Ukraine against nuclear powers in exchange for Ukraine giving their nuclear weapons to Russia in 1994. Now Russia, a nuclear power and one of the signatories to that treaty, broke it by attacking Ukraine, and the US broke it by not defending Ukraine from that aggression, because Russia is waving its nukes around as a threat.

The big lesson that the US and Russia are teaching Ukraine here is that they should have never given up their nuclear weapon. This whole event is terrible for the prevention of nuclear proliferation.