Families pushing toddlers in strollers. A group of skinny young men who say there are no jobs in their country. A young woman from Guatemala being helped by medics by the roadside, her ankle twisted and swollen after six days of walking.

These are the Central American migrants Donald Trump is sending extra troops to the border to protect America from.

On the highway between the southern Mexican towns of Pijijiapan and Mapastepec, thousands of people made their way – in groups, in couples, and some by themselves. They know Mr Trump does not like them, but many are pressing on all the same.

“There is no alternative,” said Alex Lonal, 25, a Guatemalan, whose cousin, Adelee, was being helped by the Red Cross. “There are no jobs, no work. The economy is bad. And it is a dangerous place – there are bandits.”

The size of the caravan appears to be dwindling every day. From a starting total of 10,000, it is estimated the number is now somewhere between 4,000-5,000.

The Associated Press said sickness, fear, police harassment – and offers of government money to cover the cost of their return journey – were each day chipping away at the total. Mexican officials said nearly 1,700 had dropped out of the caravan to apply for asylum in Mexico.

Carlos Roberto Hernandez, of Yoro province in Honduras, ended his effort to make it further north after developing a cough during the heat and evening rain.

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“We got hit by rain, and ever since then I’ve had a cold,” said Mr Hernandez said. Asked Wednesday if he would make another attempt to reach the US, replied: “No. I’m going to make my life in Honduras.”

But others are ploughing on, despite the difficulty of the journey and the knowledge that while many in America may sympathise with their cause, its president does not.

The caravan on the highway between the southern Mexican towns of Pijijiapan and Mapastepe (Getty)

On Thursday, it emerged defence secretary Jim Mattis expected an additional 800 members of the National Guard to arrive at the US-Mexico border amid what Mr Trump had described as a “national emergency”. The extra troops will provide what was described as logistical support to the border patrol. This would include a variety of things such as vehicles, tents and equipment.

Mr Trump said on Thursday that he was “bringing out the military” to deal with the border before addressing those in the caravan.

“Turnaround, we are not letting people into the United States illegally,” the president tweeted. “Go back to your country and if you want, apply for citizenship like millions of others are doing!”

Jose Dia, who was among a group of 20 young men from Honduras, said they knew about Mr Trump but that they would continue their journey all the same. “Perhaps it is possible to cross,” the 23-year-old Mr Dia told The Independent. “Perhaps we can make an application for asylum.”

He said the group had all come from “the barrios” of Tegucigalpa, the capital of Honduras, which, along with El Salvador, takes turns at being the deadliest nation in the world, outside of a war zone.

Gangs, drug and human traffickers, and a lack of opportunity were behind their reason to leave.

“Central Americans are travelling in groups because the journey is dangerous and there is safety in numbers,” said Alison Parker of Human Rights Watch.

"Central American migrants are fleeing severe insecurity putting their lives and safety at risk, as well as droughts and floods. Some are also leaving due to the lack of economic opportunity in their home countries.”

While Mr Trump is sending troops to the border to “protect” against the migrants, he is also weaponising them politically. His increasingly vitriolic attacks in recent days at rallies across the country is no coincidence; he knows that few things energise many of his supporters than the issue of immigration.

He has claimed without evidence, that migrants from the Middle East are among the caravan.

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He later acknowledged there was “no proof” of the claim Middle Easterners were in the crowd. But he tweeted the US “will never accept people coming into our country illegally”.

Experts say while people from countries such as Syria and Somalia do enter the US from Mexico, they do so with the help of "coyotes" or traffickers.

The caravan, which takes place every year, provides a a way for those without the means to pay a coyote, a relatively safe way to make their way northwards. The reason the journey is so long – at least 1,250 miles from the Mexican border in Chiapas – is that they are not following a shorter route that avoids especially dangerous areas they would have to pass if they went closer to the city of Monterrey and try and enter the US through Texas.

Commentators point out that the US has never been a bystander in the problems engulfing Guatemala, El Salvador and Honduras, that have led so many to leave. For decades, Washington has sided with right-wing military-led governments, suppressed freedom movements and in the case of Guatemala in 1954, ordered the CIA to help overthrow the elected government of Jacobo Árbenz in a coup. Mr Trump has not yet tweeted about this.

Melkin Claros, 34, was travelling with his seven-year old son and a teenage nephew. He told the AP that despite the challenges, he was not going to give up.

“Everyone’s objective is to arrive [in the United States],” he said, explaining that the prevalence of gangs made life in Honduras impossible.