WASHINGTON POST (NYTIMES) - A caravan of migrants, nearly all of them Hondurans, is making its way north through Guatemala toward Mexico and the United States.

It is the latest, and certainly the largest, iteration of a phenomenon that has occurred from time to time: big groups of Central Americans joining together to face the challenges of migration, their numbers providing security against the criminals that stalk the route north.

But this one has drawn the ire of President Donald Trump, who warned on Thursday (Oct 18) that he would shut down the south-west border of the United States if Mexico did not halt the group.

He also reiterated his threat that unless the governments of Guatemala, Honduras and El Salvador stopped the caravan's progress, he would suspend foreign aid to them.

"I must, in the strongest of terms, ask Mexico to stop this onslaught -" he posted on Twitter, "and if unable to do so I will call up the US Military and CLOSE OUR SOUTHERN BORDER!"

And for good measure, perhaps with the mid-term elections in mind, he took a shot at the Democratic Party, blaming it in part for the caravan of migrants.

"Hopefully Mexico will stop this onslaught at their Northern Border," he wrote in another of a series of Twitter posts. "All Democrats fault for weak laws!"

Here are answers to some questions about the migrant caravan that has upset Mr Trump this week and inflamed political concerns throughout the region.

1. WHAT IS THE MIGRANT CARAVAN?

The migrant caravan was formed late last week in San Pedro Sula, a city in northern Honduras known for high levels of violence.

It originally numbered fewer than 200 people - in line with most past caravans. But as word spread, the mobilisation quickly grew.

By the time the group had crossed the border into Guatemala, its members travelling by foot and vehicle, it had ballooned to more than 1,000.

More migrants have joined this week - by some estimates it now numbers an extraordinary 4,000 people - though it has fractured into smaller groups that were making their way at varying speeds through Guatemala on Thursday.

Like the hundreds of thousands of migrants who have fled Central America in recent years, most of the caravan's participants have in mind a new life in the United States, though some say they intend to stop in Mexico.

Many say they have been ground down by low wages, unemployment and poor public services in Honduras and are looking for better opportunities elsewhere. Others say they fear for their lives and intend to apply for asylum either in Mexico or the United States.

Advocacy groups, in the past, have used the caravans to draw attention to the migrants' plight and the state of affairs in their homelands. But no group has claimed responsibility for organising this week's caravan.

2. WHY DOES IT MATTER SO MUCH TO TRUMP?

Mr Trump made the fight against illegal immigration a central plank in his presidential platform, vowing to obtain tougher immigration legislation and build a border wall.

But he has been unable to secure financing to build the wall. And after illegal border crossings declined in 2017 to a more than 40-year low, the numbers began climbing again this year, including record-setting numbers of people travelling in families in September, frustrating the Trump administration.

During his presidential campaign, Mr Trump's fierce attacks against immigrants were wildly successful in firing up his conservative base. And with mid-term elections only weeks away, he has renewed these attacks, warning of criminals pouring over the border to threaten American citizens and suggesting that unauthorised immigrants will cause economic dislocation and job losses.

He has been using those themes at every campaign rally, which have increased significantly in the last few weeks. And many Republican candidates have embraced his anti-immigrant message in their own races.

In his caravan-inspired Twitter storm on Thursday morning, the President wrote: "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 US."

3. HOW HAVE THE OTHER GOVERNMENTS IN THE REGION RESPONDED?

Mr Trump's initial salvos in his response to the caravan this week were directed at the Honduran government, but were quickly expanded to include El Salvador and Guatemala.

He threatened all three countries with a suspension of foreign aid should they allow their citizens to continue travelling toward the United States with the intention of entering illegally. Central America has been a primary source of unauthorised migration to the United States in recent years.

With the caravan having already crossed into Guatemala, there was little the Honduran government could do to arrest its progress, though the administration of President Juan Orlando Hernández of Honduras deployed security forces to a main border crossing to try to prevent others from joining the migration.

The Honduran government also urged citizens not to join the caravan, calling it a political mobilisation aimed against the President.

Meanwhile, the Guatemalan government detained a former Honduran lawmaker who was travelling with the caravan, accusing him of failing to comply with immigration rules. And the Guatemalan and Honduran governments said they had discussed easing the reverse migration of caravan participants who decided they wanted to return home.

In Mexico, the government deployed about 700 National Police officers to the southern border and issued statements warning the caravan's participants against trying to enter the country illegally.

4. WHAT WILL HAPPEN NEXT?

Some migrants associated with the caravan began to arrive at Guatemala's border with Mexico on Wednesday, gathering in the town of Ciudad Tecún Umán. That stretch of the border, demarcated by the Suchiate River, is highly porous and is an oft-travelled route for unauthorised migrants making their way north.

By Thursday afternoon, some participants had already crossed illegally into Mexico on rafts that ply the river and had settled in temporary shelters run by the local municipality, Ciudad Hidalgo.

But hundreds of others remained on the Guatemalan side of the border to await the arrival of the rest of the caravan.

The Mexican immigration authorities have said that migrants with valid documents and visas will be allowed in, but that those who attempt to enter illegally will be detained and deported.

Those seeking asylum or some other forms of protection can request it, but will have to wait in a detention centre for as many as 45 days, officials said.

Secretary of State Michael Pompeo travelled to Panama City on Thursday to meet with President Juan Carlos Varela.

He plans then to head to Mexico, where he is scheduled to meet on Friday with President Enrique Peña Nieto, among other officials.

Migration, and the caravan, will surely be among the top issues on the agenda in Mexico City.