Even before the US polls opened Tuesday, a vanguard of immigrants at least 5,000 strong set out on a long march from deep southern Mexico to the US southern border.

The purpose: to test whether new Mexican President Claudia Sheinbaum will use the military to stop them now that the American election is over.

No less at stake in this fresh northward moving caravan challenge is whether hundreds of thousands more pooled up behind them in southern Mexico — with thousands more a day crossing into Mexico from Guatemala — will observe an unimpeded passage for this vanguard and follow it in a massive human swell that would presumably last until Donald Trump is sworn in January 20.

But Trump isn’t waiting. Just a day before the caravans launched and he won his election, Trump threatened massive, debilitating tariffs on Mexican exports if Sheinbaum lets caravans make it to the border before he gets into office.

“I’m going to inform [Mexico’s president] on day one or sooner that if they don’t stop this onslaught of criminals and drugs coming into our country, I’m going to immediately impose a 25% tariff on everything they send into the United States of America,” Trump declared at his Raleigh, NC, rally on Monday.

“If that doesn’t work,” he added, “I’ll make it 50, and if that doesn’t work, I’ll make it 75. Then I’ll make it 100.”

Recent history shows that this warning shot that Trump fired over the presidential palace has very real potential to impede any mad final mass dash on the southern border during the coming transition period — and much more.

Recall that last December, President Biden struck a backroom deal with Mexico City to alleviate the political spectacle of a badly congested southern border for the coming 2024 political campaign season. For 10 months straight, the deal has had 32,500 Mexican troops and even more federales round up tens of thousands of intending border-crossers from the country’s north and ship them by force to a militarized blockade of its southern provinces.

The operation, known in the Mexican media as “Operation Carousel,” worked wonders, cutting in half world-record illegal border crossings last fall within its first month alone and more every month since.

But no one really knows what would become of Operation Carousel once the American election was over, with Mexico feeling its obligation to the Biden-Harris campaign was now met.

Not least the thousands of trapped immigrants eager to get in before Trump takes office. They’ve been listening with growing panic to his campaign talk about closing the border down immediately while Mexico was trapping them down there and, almost certainly, his very first promissory words of Wednesday morning’s victory speech, “We’re going to fix the border.”

Trump’s tariff threat is not an idle one. Mexico’s economy utterly depends on its US exports. In 2023 and 2024, Mexico overtook China as the US’s largest trading partner, with exports from Mexico reaching their highest in the history of both countries to nearly $379 billion in 2024, increasing another 6.5% in the last quarter. Revenue from Mexican exports to the US totaled a record $593 billion last year.

That won’t be lost on Sheinbaum, a liberal progressive who has long favored passing the mass migration hot potato on to the United States.

As a protégé of former President Andres Manuel Lopez-Obrador (AMLO), Sheinbaum would most definitely recall that her old boss suffered Trump’s first tariff-threat rodeo back in 2019.

That was when Trump, facing a brief but intense surge of family units at the southern border, threatened progressive trade tariffs on Mexican exports that would reach 28% if AMLO did not deploy military to shut down his own southern border with Guatemala and hem in immigrants behind 50 militarized roadblocks leading out of the southern provinces like Chiapas state.

AMLO did as he was told to avoid economic ruination for his country.

Once Biden entered office in 2021, he swept Trump’s tariff threat stick from the table and, politely asking AMLO to keep the operation going, switched to carrots — meaning cash.

The historic mass migration of millions followed — and has now swept Trump into office again.

Will Sheinbaum heed Trump’s tariff threat? She’s being cagey so far, saying only that Trump’s election was “no cause for concern.”

“We are a free, independent, sovereign country and there will be good relations with the United States. I am convinced of this,” she said at a news conference.

The next few months will prove whether that’s true.

Todd Bensman, a senior national security fellow at the Center for Immigration Studies, is the author of “Overrun: How Joe Biden Unleashed the Greatest Border Crisis in U.S. History.”