The Wealth and Debt of Nations

Consider an international economic system in which there is relatively little trade, and then it opens up to trade in goods and to mobility in one and only one factor of production. Assume the different nations have different total factor productivities, due to technology or institutions, but that such differences are factor-neutral. What one would see is that the mobile factor would tend to move toward productive nations, increasing (even further) the marginal product of the other factors in those countries, while reducing the comparative attractiveness of the now abundant factor in those countries. If domestic "supply" of factors is at all elastic, the domestic supply of the mobile factor should decrease as its supply from foreigners surges.

I just read a snippet suggesting that it is inappropriate or confusing that the wealthiest nation on earth should have become (based on net foreign investment) a huge debtor nation. That doesn't strike me as a paradox; it just tells me that capital is more mobile than labor.