I returned from a reporting trip and discovered that China, as happens now and then, is very angry at foreigners. It began—well, I guess it really began in 1860—most proximately with two ugly episodes involving boorish white guys: one potentially criminal, the other sufficiently firable. (In one case, a drunken British tourist was captured on tape apparently sexually assaulting a Chinese woman on a Beijing street; in the other, a Russian-born cellist with the Beijing Symphony Orchestra was taped mouthing off to a Chinese passenger on a train.) But in the midst of it, the television host Yang Rui used his microblog account to unleash his international id, accusing western men of romancing local women as a disguise for espionage and calling on security officials to “clean out foreign trash, arrest foreign thugs and protect innocent girls.” He had more, and since Yang is the state broadcaster’s chosen interlocutor for foreign guests on its English-channel, writers who have appeared on the show (including me) got interested.

Then it got weirder. Right when Yang-gate should have been entering the stage of apologia, Shanghaiist translated a collection of older tweets and wondered: “Is CCTV host Yang Rui an anti-Semite?” The evidence is ugly: “Why do the US media not dare to support the call for the establishment of a Palestinian state? It’s because they’re afraid of getting fired by their Jewish bosses.” He went on to compare the head of the “US Jewish Association” to “a mafia chief,” and to point out that “Jews … control both the financial and media worlds” which leads to the “wrong biased policy of shielding Israel.”

And so on. Now and then I’ve mentioned the curiously strong feelings that ordinary Chinese folks have for Jews—feelings that are almost always cheerily admiring. It is a minor inside-joke among Jewish residents in China that revealing one’s origins is an easy way to win over a grumpy cab driver. But Yang’s implosion illuminates an uncomfortable subtext: any idea that combines so much passion with so little knowledge is volatile. Affection that is widely, if not deeply felt, has a way of changing course with the wind.

Yang’s Jewish problem generated less attention on the Chinese Web than the broader question of foreigners. It’s never difficult to find a nationalist on the Chinese Web, but the proportions surprised me this week. There is a heavy vote in support of the government’s announced hundred-day campaign to ferret out the foreigners who lack the right papers. “Whether it’s foreign or domestic, the trash should be cleaned out,” as one voice put it. “Foreigners have been treated too well for too long in China; time to get used to something new,” wrote another.

This, too, shall pass. The roots of nationalism in China are deep and raw, as I described in the magazine the last time this issue erupted. But, four years later, China continues to welcome foreigners most of the time with a quirky and endearing enthusiasm. As a snapshot of a moment, it needed to be noted. Don’t cancel your trip.

Photograph of Yang Rui by Imaginechina/AP Images.