Well, first off, legality in immigration is a cloudy term, meaning it's often tried in immigration courts, of which there are 58 in the entire country [1]. Also notable is that you are not appointed a lawyer in these courts [2]. There are only 250 judges in the country, handling more than 3 times the cases regular judges do [3]. Here's one example of an American being deported: [4].

This is the case right now. Ramping up deportation as Trump has suggested would obviously not yield better results. The immigration courts right now have a backlog of more than half a million cases [5]. I've not even touched stop-and-frisk, religious profiling, etc.

Now, and this part is speculation, this part of history comes to mind: [6]. If it's TLDR, then search for "repatriation".

[1]: https://www.justice.gov/eoir/office-of-the-chief-immigration...

[2]: http://naij-usa.org/wp-content/uploads/2016/01/2016-01-14-Re...

[3]: https://www.washingtonpost.com/national/in-a-crowded-immigra...

[4]: http://www.newyorker.com/magazine/2013/04/29/the-deportation...

[5]: http://trac.syr.edu/whatsnew/email.160915.html

[6]: http://www.npr.org/2015/09/10/439114563/americas-forgotten-h...

Edit: Bonus: http://www.theatlantic.com/politics/archive/2016/09/trumps-p...