Communist Mexico, or a violent coup against the current Mexican government. Take your pick, neither are easy to create.
Probably just a communist coup. I don't think that a more typical military coup would lead to an outright military intervention. Probably lots of hand-wringing in the press and maybe some national guard units called up and sent to the border. Maybe even some economic sanctions. But the idea of the United States intervening militarily in Mexico to restore the PRI (for a pre-2000 coup) is laughable unless the alternative was some kind of Marxist or Maoist group.
And even in the event of a Communist-inspired coup, I think the war would look a lot more like Granada than Vietnam. There would be a quick military response, with marines and special forces descending on Mexico City in short order, re-instating the old government, or some would-be caudillo who promises to "restore order" and "hold elections" in that order. United States troops would start drawing down as soon as possible, while the new leader undertakes a long and bloody campaign to repress the traitors and communists who dared overthrow the old order, probably assisted by US special forces and "advisers." Said campaign probably focuses on Chiapas and the Yucatan, with occasional flareups in the North around Sonora.
If this occurred in the 1960's or 70's, Mexico would probably resemble today's Guatemala politically, which on the whole might not be a very bad thing.