It was ended on August 21, 1968 with the arrival of Warsaw Pact tanks on Soviet leader Leonid Brezhnev’s orders followed by an occupation that crushed the reformist initiative. https://cubacenter.org/cuba-brief/2024/08/24/cubabrief-observing-black-ribbon-day-in-remembrance-of-victims-of-totalitarianism-and-the-35th-anniversary-of-the-baltic-ways-nonviolent-triumph/ 18/