[Raellus]

I don't think that you need tension between the U.S. and Mexico to justify a major arms purchase, even one including light tanks. And I don't think that the U.S. would be particularly troubled by the purchase of a few dozen, older, practically obsolete models. Yes, the U.S. might be upset that Mexico isn't buying American, but if the purchases take place after the Soviets invade China, the U.S. (gov't and arms corporations) would be too preoccupied with providing weapons to the Chinese to care.

I haven't heard much about its status lately, but in the '90s, Mexico was fighting an insurgency against a guerrilla group called the Zapatistas in its Chiapas state. Perhaps the arms buy was part of an attempt to quash this rebellion. Perhaps, Guatemala was believed to be aiding and abetting said rebels. Perhaps the Mexican government was trying to pick a fight with Guatemala in order to distract its own population from various domestic issues (poverty, corruption, etc.).

And tanks- especially light tanks- wouldn't necessarily make it that much easier for the Mexicans to overrun the School Brigade, for example. There are lots of historical examples of well-trained, experienced, well-led infantry, without its own armor, of holding off a force equipped with armor for a couple of days or three.