Donald Trump threatened Thursday to deploy the US military to close the US-Mexico border as a caravan of 4,000 immigrants fleeing Honduras in search of asylum approached the southern border of Mexico.

Calling the caravan an “onslaught” and an “assault,” Trump demanded that Honduras, Guatemala and Mexico all use armed force to stop the immigrants, tweeting:

“In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!”

Deploying the armed forces to “seal the border” poses the threat of mass arrests, mass detention and extensive military checkpoints. It raises the specter of martial law and violates the democratic principle of posse comitatus, which bars the military from carrying out law enforcement activities within the country.

The immigrants marching north have maintained a hopeful and defiant spirit despite Trump’s threats. The caravan, which is made up of people who would normally make their way separately toward the US border, has been transformed into a political demonstration whose daily movements are followed closely by all the region’s major media outlets, along with the US Spanish-language network Univision.

As the marchers, many of them mothers with young children, moved northwest en route to the United States, they chanted and waved Honduran flags and greeted onlookers, who delivered donations of food and water.

These Honduran workers and impoverished peasants are fleeing a country ravished by imperialist exploitation and US-backed death squads and thrown into disarray by the 2009 coup that overthrew the country’s elected president Manuel Zelaya—an international crime overseen by Democratic Secretary of State Hillary Clinton. In press interviews, the immigrants are gaining widespread popularity for denouncing the inequality, corruption and state violence that plague all of Central America.

The Mexican government has responded to Trump’s orders by preparing to carry out the dirty work of his administration’s crackdown on immigrants, including physical repression. Yesterday, black airplanes carrying several hundred Mexican federal police armed with riot gear arrived at the Mexico-Guatemala border. Helicopters were also deployed to monitor the border region.

The Mexican consulate in Guatemala published a statement saying immigrants who wish to enter Mexico must have “travel documents and a visa” and that those who “enter into the country in an irregular manner” will be returned “to their country of origin, in a secure and orderly manner.”

US discussions with the Mexican government have involved both the administration of outgoing president Enrique Peña Nieto and representatives of incoming president Andres Manuel Lopez Obrador, who won election in July and will take office December 1. Trump dispatched Secretary of State Mike Pompeo to Mexico City yesterday for meetings today with Peña Nieto, sitting Foreign Secretary Luis Videgaray, and his soon-to-be replacement Marcelo Ebard.

The threat to deploy the military to the border was greeted with outrage across Mexico, a country that lost half its territory to US invasion in the Mexican-American war of 1846-48 and was invaded by the US on multiple occasions during the Mexican Revolution of 1910-20.

Lopez Obrador’s incoming foreign secretary, Ebard, cynically dismissed Trump’s threat in a radio interview on Radio Centro: “The position of President Trump is the same he has always put forward, I don’t see this as anything surprising, it would surprise me if he took a different position.”

The Trump administration and the Republican Party have elevated the false “threat” posed by the caravan to whip up fascistic xenophobic sentiment in the run-up to the midterm elections, less than three weeks away. Trump also tweeted yesterday:

“I am watching the Democrat Party led (because they want Open Borders and existing weak laws) assault on our country by Guatemala, Honduras and El Salvador, whose leaders are doing little to stop this large flow of people, INCLUDING MANY CRIMINALS, from entering Mexico to U.S...”

Trump and his administration are working toward the reimplementation of a version of the family separation policy enacted last spring, when thousands of immigrant children were taken from their parents and loved ones. As Trump said during his recent “60 Minutes” interview, “When you allow the parents to stay together, OK, when you allow that, then what happens is people are gonna pour into our country.”

Officials within the Department of Homeland Security (DHS) have issued hysterical statements calling for the re-establishment of family separation.

Katie Waldman, a spokeswoman for DHS, demanded politicians “close catch-and-release loopholes in the law that would allow authorities to detain and remove family units safely and expeditiously… However, the removal of actual family units, or those posing as family units, has been made virtually impossible by congressional inaction—which will most likely result in record numbers of families arriving illegally in the United States this year.”

The Democratic Party has instructed its candidates to ignore or to publicly support Trump’s attack on immigrants in the run-up to the elections.

At a recent speaking engagement in Austin, Texas, Democratic House Minority Leader Nancy Pelosi told candidates not to focus on Trump’s attacks on immigrants, explaining that calling for “shutting down ICE [Immigration and Customs Enforcement]” merely “serves the president’s purpose.” Pelosi said she tells Democratic candidates that focusing on the issue would “waste energy.”

At an event at Harvard University this week, Pelosi proclaimed the Democratic Party’s desire to “find common ground” with Trump, explaining that Democrats “have to always try” to find ways to work with Trump. She said she thought Trump’s proposal to construct a border wall between the US and Mexico would be “expensive” and “ineffective” at stopping immigrants. She portrayed Trump’s desire to build a wall as a gender issue, explaining it was merely a “manhood issue for the president.”

Trump’s threats and the willingness of the Democratic Party to help him conduct the crackdown on immigrants underscore the sharp threats posed to the working class, immigrant and nonimmigrant alike. The ruling class is preparing to respond to the class struggle in the same way that it is moving against immigrants: with repressive violence and martial law.