If memory serves me right(no promises), the truth was that the tunnel fight was to pave the way for Alejandro and to give him opportunity to kidnap the guy who was meeting up with the middleman (forgot his name), who could lead him to the head of the cartel. The cover story was shutting down the route in order to create financial pressure on the cartel (leading Kate then to the wrong exit and realising she had been lied to).

The tunnel was an avenue to get Alejandro into Mexico to have a word w/ Mr. Big and also to take out that route.

What I never get watching this fantastic film, was Kate's objection to everything. I'd have been disappointed NOT to tag along w/ Alejandro. As Kate's handler told her flat-out, they were dealing w/ vicious scum, and 1 in 5 in the States making Drug Cartels richer than God.

Yet she continued to have issues w/ what she was doing and why, despite solid evidence that she was fighting the good fight. Come on, Kate. Get with the program. Roll that window down. And take judo lessons or something...couldn't defend herself from a crooked P.O.?

Kate was trying to do things by-the-book, i.e. properly according to the rules...

She didn't understand the ugly, cynical and expedient world she was working in... She was used... She was the patsy in this movie...