Where did we build this fence, anyway?

Wow, check out this article about the Mexican army in the US...

http://edition.cnn.com/2008/WORLD/americas/11/01/mexico.border/index.html?imw=Y&iref=mpstoryemail

I thought that we built the fence on the border, not "inside" our borders.  Isn't that giving party of our land up to the mexicans?  This occurrence of troops inside US borders is a little unsettling.  Especially when its happen almost 50 times this year.  Guess the drug lords pay pretty well.  Border Patrol has their hands full with illegals, we don't need to add military interdictions to their repertoire, do we?