A person can get in without any ID at all, if he or she can convince CBP. I know a guy who got robbed in Mexico, and came back with no wallet. Because his story was convincing, (as well as, of course, his answers to general questions about his employer, background, etc.), they let him in, after taking his finger prints and making him sign a bunch of documents.
The last few times I drove to Canada on my (Canadian) passport, the Canadian authorities asked to see my US Green Card. Of course in Canada, my green card status doesn’t mean anything, but they wanted to make sure I would be able to get back in when I went back.
That’s what I thought, but according to another thread - the fellow was born in USA, moved to Israel at age 1 and never owned a US passport - he was denied entry for a business trip because a US citizen cannot enter the USA on a foreign (i.e. Israeli) passport.
Sometimes people think they’re denied entry when they really aren’t. Choosing to voluntarily leave rather than sit in a jail cell for a couple of days while your lack of a passport is investigated is technically just a personal choice to leave, not denial of entry.
On the other hand, there’s this:
8 U.S. Code § 1185
(b) Citizens
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.
So when I drove down to the US to renew my passport, mailed my old one to the passport authority, and then left the US for Canada without a valid United States passport, I was breaking the law?
I was once given a huge hassle trying to enter the US from Canada by air back in the days when you didn’t have to have a passport. I was about 16. My mother was travelling with me. I was a US citizen, she was a Canadian citizen with no legal status in the US. They hassled me hugely, asked her if she’d heard what they said to me, and then waved her through. Ah, pre-9/11 international travel! (Edit: and although I was a minor, there were no questions asked about my father or his awareness of our cross-border travels.)
And ultimately, it’s going to be left to the border guard on the scene.
Many years ago, my American-born, Canadian-by-naturalization ex-wife found herself on a business trip to Vancouver BC. Not planning on being in Vancouver for more than three or four business days, all she had in terms of documentation, was her Canadian passport, which she always kept in her purse. The business trip had to go on longer than planned, however; and with a weekend at her disposal, she thought she might go to visit her sister in Seattle.
She drove down in her rental car, and presented her Canadian passport at the border. The US guard, seeing her place of birth was the US, gave her a stern lecture on how she really needed her US passport to enter the US–and it was stern; she said that all that was missing was the Stars and Stripes waving, while a bald eagle sheds a tear, all to the strains of “America the Beautiful” playing in the background. But in the end, her let her in.
Anecdotal only, but it does help add credence to the “it’s ultimately up to the border guard at the scene” theory.