All this talk in the thread about ancestors coming over to the US in the past is very nice - but also completely irrelevant to today. There is no comparison with those who migrate now.
100 years ago it was a big deal to emigrate halfway across the globe. It was costly, and it meant completely cutting yourself off from your home country. There was no Skype or Internet support groups. Immigrants really had no idea what they would be facing until they actually stepped off their ship. And turning back was not an easy option. There was no social support to speak of. They had to get their bearings, learn the language, learn the culture and figure out how to make money before they starved to death. Even in a best case scenario, a trip back home wasn't going to be likely until several years had passed. They wouldn't see the rest of their family for years. In other words, it took drive and some amount of courage to come to the US.
Now? They are greeted by all kinds of support groups, are constantly in touch with home via the internet, and often get nice hand-outs from their new country. Going back and forth is easy, and they are never out of touch with family. It's just so easy that everybody's doing it.
In theory, I support open borders. People should be free to travel where they wish. But they also be held responsible for their own support and their own conduct. We are not seeing that.
Talking about the past isn't useful. I can get on a plane and fly from Washington DC to the West Coast in a few hours, with cocktails and dinner served. The most frightening thing I have to face is some burly American flight attendant who hates his/her job and especially hates the passengers. Not to underestimate that, but it doesn't make me comparable to Lewis and Clark, who made the same journey with great courage and at great personal risk. Different times, folks.