The theory is called mercantilism.
The Oregon Trail was part of the idea of Manifest Dynasty because, Manifest Dynasty was the idea that America should expand into the Western Territory. The Oregon Trail is people who are traveling from the East to the West, so it demonstrates Manifest Dynasty very strongly.
The national debt is simply the net accumulation of the federal government's annual budget deficits. It is the total amount of money that the U.S. federal government owes to its creditors. To make an analogy, fiscal or budget deficits are the trees, and the national debt is the forest.
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