I was wondering if it was a glitch or not. My opening strategy is to split my army sending the Cavalry to El Paso and sending Houston and the Guards/Art to Laredo. The AI will just sit across from my 6k stack while my cavalry occupy enough land for me to get enough warscore for Mexican Texas. (I choose mexican texas for the extra prestige so I can colonize Oklahoma).
The wiki indicates that this will happen, but every time I try Mexico splits its forces. If I just occupy laredo, they sit still, but if I split they move a bit more intelligently, chase down the smaller unit, or just go around to Texas if I run from them with it.
I've also tried to get my cavalry behind the Mexican armies so they'll chase them, but the Mexican infantry, led by generals, is faster than my unled cavalry.
Additionally, U.S. chance of alliance is impossible, and they have a tendency to refuse my alliance and ally with Mexico, removing that option.
I managed to get white peace, but without Mexican Texas, and so without the prestige to colonize Oklahoma, and without the income to do anything other than become a state. It's frustrating.