Yglesias presents this map of world population density, noting that India and China have a lot of people and they're getting richer all the time. That means that in a short while their economies will dwarf that of the United States. To wax Hegelian, this means India and China will dominate the world historical stage; the story of world history will be the stories of India and China, for better or worse. This is the national greatness case for greater immigration: keep birth rates at or above the replacement rate and throw open the gates to increase the number of Americans. A nation of one billion Americans, with our head start in technology and productivity, can balance the rising powers in the east. We can then extend (what we like to think of as) our historically benevolent influence on world affairs well into the foreseeable future.
The skeptical conservative might balk at this, suggesting that opening up the gates to a bunch of Chinese is not a good way to curb China's relative influence in the world. I suppose conquest of a democratic state through immigration is possible, but it is hardly a plausible in the light of America's history. America has had for long stretches effectively open borders, and in every case incoming immigrant communities have assimilated--learned English and adopted American institutions and culture in just a couple generations. The conservative spin I suggest is this: immigration is a way of making more Americans. Few people worry now about the fate of the Republic in the hands of swarms of Italians or Irish or Germans or Japanese now. This isn't because those populations were successfully got rid of; it is because they are Americans now. For the sake of the paranoid conservative, an open immigration policy can be combined with stricter assimilation requirements as well.
In principle this applies to Europe as well. While EU enlargement isn't big on anyone's agenda at the moment, one can view admission of new nations into the EU as a way of wrapping those nations within the customs and institutions of the Europe. My impression is that much of the opposition to allowing, say, Turkey into the EU comes from fears that teeming Muslim Turks will threaten the liberal, secular society the western European nations have fashioned for themselves. But, as in the case of US immigration (or for that matter a North American economic union), the current EU nations have a better bargaining position and are more likely to make Turkey more secular and more liberal than vice versa.
