Christopher Hitchens provides an answer to a long-standing question of mine in his article "Jefferson Versus the Muslim Pirates." He quotes James Madison:
"It is a settled policy of America, that as peace is better than war, war is better than tribute. The United States, while they wish for war with no nation, will buy peace with none."
Then Hitchens adds: "The expression 'the United States is' did not come into usage until after Gettysburg."
I've always said that there is a great linguistics paper to be written on this topic—perhaps it has been written already.