Great question — answering as the UphillTech team:
The US law ( 8 USC 1185(b) ) says this: Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport. You don't have to present it to anyone unless asked to, but the US law does require you to have it when traveling internationally. 22 CFR 53.2 lists several exceptions, some are relevant specifically to travel to Canada (using trusted traveler program cards, passport cards, etc, as substitute). Usually, entry controls require...
Full answer by oh whatever: https://travel.stackexchange.com/a/195469 (CC BY-SA 4.0).
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