Question the united states was sometimes called the "world's largest debtor" because

Foreign investors owned a greater amount US stocks, bonds, and factories than investors in the US owned of assets in foreign markets.

In 1985, the New York Times reported, "U.S. Turns into Debtor Nation," because a Commerce Department report showed the US "owing foreigners more then they owe it."  By that they meant that "foreign ownership of American factories, real estate, stocks and bonds exceeded American ownership of foreign assets."   
However, there's another way to look at this picture than the "debtor nation" label.  The Heritage Foundation (a conservative group) noted in 1985 that having foreign investors pursuing assets in the United States indicated strong confidence by those investors in the American economy.  You invest in a country's assets because you think those assets will grow in value.  So, becoming a "debtor nation" can be viewed as a sign of economic health in the eyes of the rest of the world.

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