Back in 2004, Congress approved the Western Hemisphere Travel Initiative. It dictated that in order to cross the border back into the country from either of the United State's two neighbors, a passport would be required. Today, that law goes into effect.

Beginning today, Americans returning from Mexico and Canada must show valid passports, a sharp departure from previous regulations that let U.S. citizens use only a driver license as proof of residency.

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Now, any U.S. resident 19 and older will need to show border agents a passport, one of several forms of federal identification, or a combination of two documents, such as a driver license and a birth certificate, before entering the country by land or sea. [Daily News]

Feeling a bit landlocked? The Daily News says that to "try to ease the transition, U.S. citizens will be able to preorder driver license-size passport cards from the Department of State beginning Friday." This "Passport Card" will only be valid for land and sea travel (note: no air travel) between the U.S. and Canada, Mexico, the Caribbean region, and Bermuda, according to the State Department.

Image from the State Department