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ALL PERSONS traveling by land and sea outside of the United States are required to present a passport book/card, or other valid travel documents to enter or re-enter the United States.
ALL PERSONS traveling by air outside of the United States are required to present a passport book or other valid travel document to enter or re-enter the United States.
More Information: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html