Border crossing back to San Diego for a non-US Citizen

In theory you'd be fine crossing back from TJ on a B1 or B2 visa.
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And in practice, PedEast at San Ysdrio is the largest and busiest port of entry in the world. They don't have time but to spend a few seconds on each traveler.
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With that said, I've read a ton of fucked up shit about border crossings for non-citizens lately. Interrogations, searching and confiscation of phones, denying entry and rescinding visas. See: nbcnews.com There was a recent article about the fucking governor of Baja California and her spouse, whose Visas were revoked when they tried to cross the border. See: nbcsandiego.com.
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I hate that this is true, but one key factor is: are you a white guy from a predominantly white nation? This administration seems hellbent on policies of cruelty, terror and uncertainty for anyone brown, born of nothing but basic racism and xenophobia. Another key factor is: are you an outspoken critic of this administration? This administration is also hellbent on seeking retribution for it's political opponents. Most likely, if you can answer "Yes" and "No" to these questions you'll be fine. I fucking hate that that's how it is but it do be like that right now.

Thank you for the detailed response. Given that my answer is No to the first question, I guess it's not worth the risk...

Ah, dude, that absolutely sucks and I'm sorry for you. I fucking hate what is happening in the US rn.
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I don't want to be the sole source for your decision so do your research obviously, but I'm sure you'll find out what I'm telling you is true: it's risky for you. See this travel warning mintz.com for a much better source. I will note that this source does say:
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The vast majority of individuals who are authorized to enter the United States are being inspected and admitted without issue.
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And like I said before, PedEast is the busiest port in the world. They don't have time to be fucking with everyone. However, it's not without risk and you can read the travel warning for yourself so you understand the risks and can proceed accordingly.

One thing you could do that would up your chances of not getting fucked with at the border: apply for Global Entry.

Yeah, I considered Global Entry, but unfortunately I don't have enough time to apply for that now, given that I'm traveling next week. Ah well, I guess it was good while it lasted - I visited HK 3 times, though the last visit was almost 7 years ago! At that time, when I re-entered the US, they didn't even ask me what I was doing in TJ!

I am a US citizen but my friend is not. He is still undecided about traveling across the border but what is a good answer when asked what we went to TJ for?

Usually a very curt answer here is all that's necessary. "For vacation."

But if pressed you could say you spent your day(s) at the beach in Rosarito and night(s) getting cocktails and food at Ancor restaurant in Zona Río.

@wishy007, I should also add that CBP doesn't care about PLs mongering in the Zona. It's not illegal to do so (yet.) PLs report all the time telling CBP they went to the Zona and they don't care.

What I would be concerned with for your friend is whether he has done or said anything deemed "unpatriotic" by this administration. As I mentioned earlier in the thread there have been numerous cases of immigrants being detained, interrogated, and even denied entry for specious reasons, seemingly their politics.

If your immigrant friend is a big online activist for Gaza or anti-Trump causes, I would proceed with extreme caution.

The most likely scenario though is that he'll be fine, so long as his passport and visa documentation are valid.

Thank you for the detailed answer @ronjax2. We will proceed with caution. My friend is only visiting the US for 2 weeks on a business visa (B1/B2) for a company event in San Diego. His port of entry would be SFO a week prior.

@wishy007 - NP. I'll wager there's a >98% chance he'll be fine. Like I said earlier in the thread, there's so many people crossing at that port of entry, they can't fuck with everyone.

And to me, at those odds, it's definitely worth the gamble, because HK is otherworldly. If I were your friend I'd still go!