The only thing I could come up with is that the rift between Cuba and Mexico increases after the rhetorical dispute between the two nations that started in 1998(when Fidel Castro said that it'd be easier for Mexican children to name Disney Characters then Mexican Historical figures). In OTL, Mexico recalled their ambassador and asked the Cuban Ambassador to leave. In response Cuba ousted some Mexican busniessmen. However, it all cooled down when Castro and the Mexican President, Vincente Fox, apologized to each other around 2002. However, lets say it gets worse, and Fox uses it to push the new
Castañeda Doctrine, and takes a hard stance on Cuba, wanting to put Mexico as a bigger player on the world stage. By 2003, the rift hasn't closed between the two nations and Fox has increased the size of Mexico's military somewhat. In 2003, the USA gets tied up in Iraq, and the drug trade in Mexico becomes a worse and worse problem, destabalizing the country. As the Fox government comes up for re-election, the Mexican Inteligence Agency(or the American CIA tells the Mexican government) that some of the planes smuggling drugs are coming from Cuba. Relations collapse after a Cuban fighter accidentally collides with a Mexican planes over the Caribbean, and the Mexican government thinks it was an attack and declares war.