I was reading this book called Prisoners of Geography and it says that Mexico isn't really attractive from a geographic point of view. If you think about it, the north is a desert, the south is a jungle and central Mexico is full of mountains. I don't know if the country is very well connected and how easy it is to travel, etc.. Argentina for example is pretty flat. Chile has the central valley where most people live, but it's still got an isolation problem. So yeah, I don't know if Mexico has had to "held its own against the US" really because I'm not even sure Mexico is attractive for the US to start with.