250 miles sounds pretty specific. Without giving away your source, did more than one person tell you the same details?
I just thought the 250 miles and the entire story was part of some wild rumor that had been created.
The guy that convinced me it wasn't just a wild rumor got the story from his family in Mexico while Carter was still president.
He was Mexican and was married to a white girl. He had never been to Mexico in his life. He and his wife went to Mexico to visit his family that he had never met. Because his wife was white, nobody thought she spoke Mexican, when in fact she spoke Mexican better than he did. Every place they went there was talk of the coming revolution. He was able to pick up part of what was being said, but his wife picked up a lot more. Scared the living crap Ont of her.
The guy said his family was also very open in talking about the revolution that was planned and how it was planned, with Carters help, to take back the southern part of the US for Mexico. His wife again stood by and played dumb like she didn't understand what they were saying.
Until this guys family in Mexico told him the story of the planned revolution and the taking back of the southern US, he too just thought it was a wild rumor. The guy and his wife left Mexico and said he would never set foot in that country again.