In my opinion the easiest way to do this would be during the Mexican Revolution. Let's say the Mexican Revolution goes much like OTL. Madero is Assassinated in 1913 by Huerta, but But afterwards we get a different POD where Huerta is Subsequently assassinated by Zapatistas. Zapata emerges as the de facto leader of Mexico by 1914 Which subsequently terrifies the United States. Zapota is then assassinated with suspicion falling on the United States. Three straight Presidents of Mexico assassinated in two years radicalizes the already unstable country, and extreme elements of the Zapatista movement emerge. By 1915 the Zapatistas are in firm control of southern Mexico (where Zapata hails from), while Felix Diaz (Thanks to Clear support from the United States) Is infirm control in northern Mexico. With the outbreak of World War I, the United States decides having an Unstable neighbor to its southern border is not in its best interests, so it forces and uneasy peace treaty between the two sides. Northern Mexico remains a close US ally, while southern Mexico emerges as the world's first communist state. Although the United States does not like having a communist nation in the Western Hemisphere, in 1915 it does not quite take the communist threat as seriously as it does in later years. By 1920 the People's Democratic Republic of Mexico (Or South Mexico as it's referred to) Establishes close ties with the new Soviet Union. This makes Mexico the frontline of the soon to emerge Cold War between the United States and the Soviet Union. South Mexico exports it's brand of communism to Central American nations like El Salvador Guatemala and Nicaragua, Wow coaster Rica Panama and Honduras strengthen ties to the United States