In 1968 the situation for the Mexican government was unprecedented, it was the biggest protest yet in modern history as students and citizens protested the authoritarianism of the ruling PRI party.
In that era, the president was basically a 6 years term dictator.
What is however largely unknown, is that there is evidence the US was actually worried that a revolution could happen, which would be a trouble in the middle of the Cold War, right at its doorstep.
The situation got so bad, that there are reports the US ambassador called the Secretary of Defense and gave him green light to launch a coup against the Mexican president, to calm down the population. The Secretary refused the petition and instead, the army was sent to slaughter the protesters.
The action was successful, while this forever tainted the PRI and government image in the collective memory, it managed to subdue the protests and end the movement.
However, it is easy to see, the situation could had gone in the opposite direction, if the slaughter rather than scaring the protesters made the opposition even more fierce and it grew into a full scale revolution. It is interesting to note, that among the protesters, there was a big acceptance to communism, which means that the Soviet Union would likely be quite interested.
What would had happened if there had been a revolution in 1968? Could it had become another Cold War scenario with the US and the USSR backing different sides?