I wouldn’t put it that way exactly. Ukraine was a constituent part of the USSR, and eastern Ukraine was a major industrial hub. I don’t know the history of it, but perhaps giving the nukes to Russia was part of the deal made in the Post-Soviet transition in Ukraine.
Ukraine was very much at peace with Russia as it always had been before the 2014 coup and the resulting fascist gov that didn't care about such agreements.
The region doesn't rely on the US, the post-2014 fascist government that doesn't care about the wishes of eastern Ukrainians does.
There wasn't a problem here until the US-backed coup in 2014 and subsequent killing of eastern Ukrainians and Crimeans (if you're wondering why Crimeans overwhelmingly support Russia) by the coup gov for resisting, which also doesn't respect the Minsk agreements to stay neutral.