I honestly think it would have been more humane to have tightened the southern border in the 50s and 60s before we stole so many of their ambitious and politically discontent young people for our labor force. If they had stayed home, one might fantasize that they would have forced changes in their home countries.
I've watched the changes in the cross border traffic and in Mexico over the past 30 years.
In 1986 when I was transferred to Dallas - the vast majority of illegals working in Texas were partial year residents. They came across the border in spring, worked all summer, sent wages home to wife and kids, and returned home to Mexico in October or November. They lived three or four months each year with their families in Mexico. They didn't put up with drug dealers, gangs, 'cartels' making the homes of their children a war zone.
As it got tougher and more expensive to cross the border - those workers did the math. Realized it was cheaper to move the family to the US. Once established, the risk of standing out and being 'caught' was minimal.
That resulted in the smartest, brightest people leaving those towns and cities - and leaving them to the control of the undesirable elements.
I won't put all the blame for the 'cartel' ownership of many parts of Mexico on the 'tightening' of the US border - but a substantial part of that effect is due to the US efforts.
Another impact was the changes that President Reagan got passed with his 'tax reduction' laws. That made it possible for US corporations to replace US citizen blue collar labor with 'contract' labor - often non-US citizens.
This is the biggest job loss for the US citizens. Before Reagan, if a meat packing plant had illegals working in the plant - the plant manager and corporate owner were liable to INS fines, and possible criminal charges - if the employees worked for a 'contractor' - it was still the responsibility of the plant management to ensure all were legal to work in the US.
After Reagan's changes - the plant management / ownership has absolutely no responsibility, or right, to require / verify - that all contractor employees are legal to work in the US.
Before Reagan - most illegals worked on a cash basis in very labor intensive jobs.
Total - the vast majority of illegals work for contractors. They get paid by check, have Social Security and IRS withholding from their paychecks (yes they take the maximum amount of exemptions to minimize the FITW). The result is that Social Security and the IRS get billions of dollars each year - and never have to issue tax refund checks and will never have to pay social security benefits to those people.
The people in the US Congress screaming for a closed border know that the solution is very simple. Make it impossible for those without proper authorization to work in the US. Take them out of the plants, construction industry, etc - by requiring positive identification, and making the management of the facility/ job responsible. Stop the 'contractor' loophole. There is no need to 'close the border'. The need is to eliminate the jobs. Last year (2015) the US had a net negative migration from Mexico (not including Central/ South American illegals). More illegals moved back to Mexico than came across the border - because of the cutback in oil industry jobs in Texas. We don't need a wall to stop illegal immigration - taking away ability to work will do it better and cheaper.
The really funny part of the "We will build a wall" promise is that the project would be the biggest employer of illegals in US history.