Recently I got into a debate on open borders with respect to trade and labor. I am in favor of free trade between countries and also open borders wrt labor, almost all schools of thought are in accordance with this theory. Monetarists, Austrians and even Keynesians all agree. In a nutshell, the theory states open borders will lead to increased economic activity and increase the standard of living.
The person I was debating was a fan of Adam Smith and was claiming that Smith was against open borders to trade and labor, or at least as far as I could tell. He thought that open borders to trade and labor, and commodity backed money would lead to "mutually assured economic destruction". I have read some Adam Smith, probably not as much as I should have, but I have read a little more since the debate and I found out that the person I was debating had it completely backwards.
I was surprised to hear someone claiming that Smith was against open borders in the first place. I knew he was in favor of free trade and I could not imagine him being wrong on this. So I read into his views on borders and one quote sums up his view very well. "The core of free trade, is the free circulation of labor". Clearly this implies that Smith opposed any mercantilistic restrictions with regards to not only trade but labor as well. It makes sense that he would be in favor of open borders, every good economist I have ever read or spoken with is in favor of it. Furthermore, it is almost all economists pet peeves to hear people talk about why tariffs are good and we can't let immigrants steal our jobs etc. etc.
I had no idea what Marx thought of immigration at the time of the debate but he sent me a link of something that said Marx was in favor of open immigration to reduce national identity. This sounded like something Marx might say, but my research did not agree with this assessment. Basically, Marx was against any immigration thinking it was a ploy of the bourgeoisie to bring in cheap labor to drive down the wages and further exploit the labor. It also makes sense that Marx would be against it. He is one of the worst economists in the worlds history.
Disclaimer: When economists talk about open borders to trade and labor they are not endorsing letting anyone in the country at anytime. Rather, they are endorsing making it easier for people to enter a country for jobs if they are demanded.