MEXICO CITY >> President Donald Trump threatened today to scrap a crucial trade deal and send soldiers to close the U.S. southern border in response to a large caravan of Central American immigrants now heading toward the United States.

In a series of tweets, Trump said that if Mexico is incapable of stopping the immigrants, he will “call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!”

Trump also threatened to upend a pending trade deal with Mexico that took more than a year to negotiate and is designed to replace the North American Free Trade Agreement.

“The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA,” Trump said, referring to the United States-Mexico-Canada Agreement.

His threats came as hundreds of members of the loosely organized caravan arrived at the Mexico-Guatemala border. Mexican authorities have closed a border crossing near the town of Tapachula, and have sent hundreds of federal forces to the area to help immigration authorities guard popular illegal crossing points.

Hundreds of other members of the caravan who are lagging behind are expected to join the group that is gathered at the border.

It was unclear whether the immigrants would try to rush the border, as they did at the Honduras-Guatemala border several days ago, or whether they plan to turn themselves in to border authorities with the hope they will be granted entry.

Mexican authorities have said that members of the caravan will be treated like anybody else seeking to enter Mexican territory: Those with proper documentation will be allowed to pass while anybody entering the country “in an irregular manner” will be apprehended and, if appropriate, returned to their home country.

Immigrants fleeing violence or other threats who seek refugee status to stay in Mexico will have to request it at the border and wait up to 45 days in immigration detention, the government said in a statement Wednesday.

The last time a similar caravan of immigrants sought to cross into Mexico, in April, authorities gave its participants short-term visas that allowed them to travel through Mexico and reach the United States border. That angered Trump, who sent National Guard troops to the U.S.-Mexico border, saying the caravan posed a threat to U.S. sovereignty.

Trump did not elaborate on his threats to send soldiers to the border today. Along with his warning that he is ready to engage the military, Trump has also threatened to withdraw aid from countries that fail to stop the caravan.

The caravan, which includes people fleeing violence and political repression as well as migrants in search of jobs, departed Saturday from the Honduran city of San Pedro Sula — two days after Vice President Mike Pence urged the leaders of Honduras, El Salvador and Guatemala to persuade their citizens not to enter the United States illegally.

Recent U.S. efforts to curb immigration have come as record numbers of families have sought to enter the United States, although the overall number of people crossing the border is much lower than in previous years.

Secretary of State Michael R. Pompeo will visit Mexico City on Friday to meet with Mexican President Enrique Pena Nieto. The caravan will be on the agenda, a senior State Department official told reporters Wednesday.

Trump, who campaigned for president with vows to stop illegal immigration, said on Twitter today that he thinks Republican candidates for Congress should campaign on the issue in the upcoming midterm elections.

“Republicans must make the horrendous, weak and outdated immigration laws, and the Border, a part of the Midterms!” he said.

Many immigrants said they joined the caravan after seeing it covered by Honduran news outlets last week. Like previous caravans, this one was designed to help protect immigrants from the dangers of the migrant trail, which include robbery, rape and assault perpetrated by smugglers, cartel members and sometimes immigration and law enforcement authorities.

One participant, Dilmer Vigil, said earlier this week that he joined the caravan because he had been unemployed for six months.

“We are not leaving because we want to,” he said.

Vigil said he believes Trump is partly to blame for the recent exodus from Honduras because of U.S. involvement in the country’s contested presidential election last year.

Initial election results showed opposition challenger Salvador Nasralla with a 5-percentage-point lead over incumbent President Juan Orlando Hernandez. Then the electronic results system went down for hours. When it came back online, Nasralla’s lead had disappeared in favor of Hernandez, whose claim to victory set off months of opposition protests and violent crackdowns by the government.

Despite claims of election irregularities and calls for new elections by the Organization of American States and others, the U.S. recognized Hernandez as the winner.