No, U.S. citizens did not always need a passport to enter Canada; for decades, a simple driver's license or birth certificate was sufficient for land and sea crossings. This changed significantly following the 9/11 attacks, leading to the implementation of the Western Hemisphere Travel Initiative (WHTI). The first phase required passports for air travel in 2007, and the final phase, implemented on June 1, 2009, mandated that all travelers (including U.S. and Canadian citizens) present a passport, passport card, or enhanced driver's license for land and sea entry. This shift marked the end of the "open border" era for casual travel, though exceptions still exist in 2026 for children under 16 using birth certificates at land crossings.