Americans flying to Canada, Mexico and the Caribbean now need a passport to get back home.
Starting Monday, the U.S. government requires a passport or proof of citizenship, such as a birth certificate, for all air travelers arriving from countries within North America.
The rule had gone into effect in January. But the State Department suspended it when it could not keep up with the demand for new passports.
Previously, air passengers arriving from nearby countries had to show proof of citizenship only at the discretion of border agents.
Starting next year, Americans traveling by sea or car within North America will also need to show proof of citizenship.
Travel to and from other continents has long required passports.
Americans who left the country before the rule went into effect can return without showing a passport.
The passport rule is part of the government's Western Hemisphere Travel Initiative.