Were they? And is that the justification the US has cited? If not, you're writing fan fiction and that's not really interesting.
I'm not a supporter of totalitarian regimes including Maduro's, but the US has a track record of producing very poor outcomes for people in South America when they topple one leader in favor of a more--shall we say--"market friendly" character waiting in the wings.
As for international law, it is extremely clear, prohibiting the threat or use of force against the territorial integrity or political independence of any state. International law recognizes only two clear exceptions: self defense or a US Security Councul resolution.