Castro did that. Google Mariel Boatlift of 1980.

But the US is not in decline because of whatever anyone from outside does. It's following the same cycle all Hegemons go through over 100-200 years. Whether its Greece, Babylon, Eygpt, Rome, Islamic Caliphates or all the later European powers. They all went through a similar a cycle - rise - dominate - decline. See Oswald Spengler - Rise and Fall of the West written 100 years ago.

You're referring to history, which nobody gives a care about it seems. Here in the US, it's as if we're living in a bad sci-fi/horror movie the last ten years. People argue about politics, forgetting to hold politicians accountable to any laws. Most of SCOTUS is a party stooge and the POTUS is a mafia type thug, basically blackmailing corporations and law firms. Trouble is, this "cycle" will most likely have world wide repercussions and in a lot of cases already has.