> Which ratified treaty did the US's operation in Venezuela violate?

Even if it hadn't violated a ratified treaty (it did violate several, starting with the UN Charter and OAS Charter), it would still violate international law; the US has recognized (among other places, in the London Charter of 1945 establishing the International Military Tribunal) that the crime of aggressive war exists independently of the crime of waging war in violation of international treaties.

The UN Charter is a rather unambiguous one.

The US agreed in Article 2 of the UN Charter, which they ratified on July 1945, that they would refrain from the use of force against the political independence of any state.

The reason you rarely see people cite the exact provision is that it's pointless to cite, because the US foreign policy establishment does not care and will not be swayed by persuasive arguments about their treaty obligations.

> operation

Putin has one too.