POD: The Treaty of Guadalupe Hildago established the USA-Mexican border west of the Rio Grande to be the 31 degree 20 minutes parallel from the Rio Grande to the Sea of Cortez.
Seems reasonable. As has been pointed out before on this forum, while annexing the Mexican heartland was unlikely, it's a bit surprising the US didn't annex more of the at-the-time rather thinly populated north of Mexico.