Blatant violation of the Vienna Convention. This is an issue worth going to war over.
I think it was the Russians. I say that because Cuba and Russia have long been tight allies with Cuba being the Russian close proximity to mainland USA and having a land base on Cuba; probably covertly.
Russia likely doesn’t want Cuba and the USA to be friends or even get cozy. Trade would be about as far as they would like it to go so Cubans could finally get the finer more modern things in life.