North Western most Mexico and the South Western most USA - next letter "A"
(btw - this area of the world is called "Friendship Park" where families separated by countries not long ago could sit and have lunch together despite being separated by a fence. Sadly, access to this fence is now only allowed between 10:30 - 2 PM Sat and Sun and even more sadly they've installed fine mesh fencing and now it's difficult to even see one another. Don't get me wrong, I understand the need to keep the a country safe from terrorist but there comes a point of diminishing returns when you take this mission to the extreme)