BROWNSVILLE, Texas - If you want a short course in understanding why border relations between the U.S. and Mexico tend to be a bit shaky, I heartily recommend a visit to the place where the border wall begins. That’s in this city at the eastern end of the Rio Grande near where it empties into the Gulf of Mexico. To get to the wall, which was built during the Bush 43 administration, you drive to the campus of Texas Southmost College and then follow a dirt road as it runs along an eight-foot fence. The fence soon turns into a...