[Raellus]

Zimmeran Telegram c.1917 = Zhukov Telegram c. 1997?

Might the Mexicans just be trying to take back some or all of the territories they lost to the U.S.A. in the mid-to-late 19th century?

Considering how hard pressed the U.S. already was when Mexico invaded, and adding in the already prodigal use of nuclear weapons in other theaters (as James already pointed out, using small nukes probably didn't seem like such a big deal anymore), and recognizing that Mexico didn't have the capacity to retaliate in kind, a tactical nuclear strike against at least one major road/rail node in Northern Mexico makes a lot of strategic and operational sense. It would blunt any Mexican drive but seriously impeding their logistical capabilities (already pretty sketchy), as well as sending a strong message to the Mexican government. And it would allow the American government, such as it is, more time to organize a strong [enough] conventional response.