Our immigration system is COMPLETELY BROKEN.
Even for wealthy, well educated immigrants with family members in the USA, it can be a Hell of a goat screw to get access to the USA legally.
For poor laborers, the chance of entering the USA to work legally is essentially nil. If you have workers looking for a job, and US companies willing to hire them, but the US immigration system is so completely F-cked up neither the employer nor the potential employee has any chance to work legally, within the system, then it doesn't take a genius to realize that they will IGNORE THE SYSTEM and enter illegally.
My wife is a LEGAL immigrant. Even with money, education, family in the USA, and an in demand job skill (scientist with a graduate degree), she had to jump through ridiculous hoops to become an American citizen.
If you make working within the system legally nearly impossible, don't be surprised if people IGNORE THE LAW.
First step should not be putting machine guns/troops on the border, it should be FIXING OUR BROKEN IMMIGRATION SYSTEM.
I'm no fan of George W. Bush, but one of the few bright ideas he had was setting up a system of work permits for foreigners. That would have been a great first step. Of course, that idea went nowhere, whereas stupid ideas like the Iraq War were endorsed by BOTH major parties. Utterly pathetic.