First questions is, I keep reading that The Ukraine had gone back on an agreement they made with Russia, is this correct? (if true I’m sure it’s not a simple yes/no answer
it's the other way round - Ukraine gave up the Soviet nuclear weapons on its soil because it could not maintain them (and there was a worry around the world that Soviet weapons could fall into the wrong hands after the break-up of the USSR). In return, Russia, the US and the UK (and others?) agreed to protect its sovereignty. Not much evidence of Russia meeting its side of the bargain.
There is some talk of NATO promising not to expand eastwards, but even if that was true, it wasn't for those at the time to tie the hands of sovereign states later - and ex-Soviet satellites such as Poland and ex-USSR-states such as Estonia joined.