William O. Douglas,
Kent v. Dulles — Opinion of the Court
“ It is a document, which, from its nature and object, is addressed to foreign power; purporting only to be a request, that the bearer of it may pass safely and freely; and is to be considered rather in the character of a political document, by which the bearer is recognized, in foreign countries, as an American citizen; and which, by usage and the law of nations, is received as evidence of the fact.'A passport not only is of great value-indeed necessary abroad; it is also an aid in establishing citizenship for purposes of re-entry into the United States. ”
