[kabar]

A large population is helpful not only economically, but also militarily. Ideally 100 million armed American citizens would be able to repel an invasion, but they'd still be weaker than 1.5 billion armed Americans.

Nonsense. Why do you think China and India are trying to reduce their population growth? If 1.5 billion Americans seem like a good idea to you, how about 3 billion? The USG's pro-population growth policy expressed thru legal immigration should be put to a vote by the American people. Do we want a population of almost 500 million within the next 60 years? I don't, especially given the problems associated with assimilation and massive demographic shifts.

[Jedi Master Pikachu]

Off topic, but if space colonization was made feasible, and the United States expanded to other planets, would support a population in the quadrillions or larger.

For Earth, a population large enough to take on the rest of the world combined (which is also why support Canada and Mexico becoming part of the United States--more land and resources comparable to the rest of the world, instead of the roughly (and relatively paltry) 1/16 of the world's landmass the United States covers today).