What document is required for U.S. citizens crossing the border into international cities?

A passport or passport-replacing documents are necessary for U.S. citizens who wish to cross the border into international cities. This requirement is in place because a passport serves as a government-issued identification that verifies a person's citizenship and identity when traveling internationally. It is also crucial for compliance with customs and immigration regulations.

While a driver's license is a valid form of identification domestically, it does not meet the requirements for international travel. Similarly, a birth certificate, while it can prove citizenship, is not an accepted form of identification on its own for crossing international borders. A Social Security card, although it verifies a person's Social Security number, does not establish identity or citizenship for international travel purposes. Therefore, having a passport or acceptable passport alternatives is essential for any U.S. citizen traveling across international borders.