What? Are you under the impression that the millions of illegals are all hiding in Wopaw, Arkansas or something? And the army is going to go in and round them all up for processing like that scene in the original Red Dawn?
They're all over the country. ICE has been rounding up and deporting illegals for a long, long time before Obama made it cool to leave them alone. Once the wall is built, you find the illegals through tightening work documentation, deporting criminals, etc. There are processes for this, and a department that is very good at it, if it is left to do it's job. Will you get them all? Of course not. But that doesn't mean you ignore the law and the problem.