Let's say I forgot my US passport in my home in Canada.?
By US law, American nationals must enter the US on a US Passport ( or similar document like a passport card) or in some cases show proof of citizenship ( naturalization certificate) when entering the country. Dual Nationality | U.S. Embassy & Consulate in Spain and Andorra INA: ACT 215 - TRAVEL DOCUMENTATION OF ALIENS AND CITIZENS Edit: As Kathryn Berck states, even an American with no documentation would eventually be let in, albeit with some delays while CBP confirms your identity and that you are a citizen.