Sergio Pérez, who's from Guatemala,

is attempting to migrate to the US.

The biggest challenge used to be getting across the US border.

But now, Mexican President Andrés Manuel López Obrador has

made it increasingly tough even to get into Mexico.

Since June, after Donald Trump threatened

to impose tariffs on all Mexican exports,

he has clamped down hard.

The numbers show the policy is working.

There has been a more than 75 per cent drop

in the number of migrants making it into the US.

And Mexico had deported as many people

by October as in the whole of last year.

But the Mexico-Guatemala border is hard to police.

It runs through mountains, jungles, and rivers

for nearly 900km, and is full of clandestine crossings.

I travelled along it to see the impact of López

Obrador's crackdown.

I wanted to try to understand what

keeps driving people like Sergio to leave

when the chances of success have narrowed dramatically.

And to find whether Mexico's southern border

is becoming a proxy for Trump's border wall.

Breaking right now, everyone, President Trump

saying that the US is prepared and is at this moment in time

preparing to impose a five per cent tariff on all goods

imported from Mexico if that country

doesn't start to do more to combat the migrant crisis

on our southern border.

Mexico's president had at first welcomed

Central American migrants fleeing poverty and violence

and offered them humanitarian visas,

but many use them just to travel unhindered through Mexico

to get to the US.

Washington has gotten tougher, signing deals

with Guatemala, Honduras, and El Salvador to force migrants

to seek asylum there first, not in the US.

It's a very big thing.

It's very important signature.

Never been done before.

But those deals will be hard to enforce

and the pressure is on Mexico to stay tough as next year's US

election approaches.

Mr López Obrador knows that any misstep on the issue

of migration could blow up into a bilateral crisis.

I think that this is going to be a stone

in the shoe of the Mexico-US relationship

for the rest of the Trump administration and probably

whoever comes next, because migration

is a very sensitive issue in the United States.

And it's suddenly becoming a sensitive issue in Mexico.

López Obrador is in a tough spot.

He regularly hails Mexicans who migrated to the US

in past decades and send back record remittances

as living heroes.

But he's put the need to protect Mexico's relations with the US

first.

Do not pay the smugglers.

Do not pay the coyotes.

Do not put yourself in danger.

Do not put your children in danger.

Despite the dangers and increasing difficulties,

the people smugglers are still finding ways to make a profit.

Here in the remote highlands of Guatemala,

I've come to meet one of the go-to-guys

in this illicit business to find out

what has changed in the past few months

since the crackdown began.

Alex is a coyote, and he's been smuggling people to the US

for the past seven years.

He says things are getting harder.

Alex charges $7,500 and says he makes a profit of about 600

or 700 dollars per migrant.

But that's after paying $800 per person to a drug cartel

to deliver them to the US border.

Sergio is willing to chance everything.

But his job in his shop only pays $5 a day

and he's only been able to muster $1,800.

So he gives up the only thing he has

- the deeds to a plot of land.

If he doesn't pay in two months, it will belong to Alex.

While Sergio was setting out from the town of Cuilco

along this bumpy single track road headed for one of scores

of informal border crossings high in the mountains,

another migrant family was just crossing into Mexico.

The Marine Martinez family was forced

to flee from Honduras after their 18-year-old son was

murdered and their home burned to the ground by local gangs.

One of the easiest ways into Mexico

used to be across the Suchiate River that makes up

part of the border with Guatemala,

but now, every person is being stopped

by immigration officials something that's

never really happened before.

The Marine Martinez family is trying a different route,

but it's along a main road, where

police have set up roadblocks.

It's already their second attempt

and they are terrified, running into the bushes

at the sight of a police car, even though local police

tell us they have no interest in migrants,

and it's the federal police they need to watch out for.

Finally, Father Tomas, founder of the local migrant shelter,

arrives to rescue them.

Here, 750 miles from home, they're

safe, and will receive medical care and legal advice.

The children are exhausted and famished.

One of the girls is sick.

Ramón Márquez runs the shelter.

It's called The 72 in honour of 72 migrants murdered

by the drug cartel, the Zetas, in 2010.

Donald Trump's tough stance has also

been boosted by a Supreme Court ruling in September that

makes it virtually impossible for Central Americans

to apply for asylum in the US.

He has also been pushing asylum seekers back over the border

to wait for their court hearings in Mexico.

But Ramón says that with homicides in Mexico at a record

of some 34,000 last year, the country just isn't safe enough

to take them in.

Mexico's migration authorities, which have suffered big budget

cuts under López Obrador's austerity government,

are struggling with the sheer numbers.

And even though Mexico has a long history

of taking in refugees from civil war and dictatorships

in Spain, Guatemala, and South America,

polls show that public opinion is increasingly

hostile to migrants.

Mexico says the answer is development.

It wants Washington to help fund social programmes.

It's the right diagnosis, but it will take years.

And as Trump cuts aid to Central America,

getting the US to cough up may be hard.

Sergio's American dream quickly turned into a nightmare.

He was in a group of about 11 people trying

to cross the desert.

Nine made it, including his companions from Cuilco.

He and two others were deported.

Back in Guatemala, worried about all the debts he'd racked up

and the fact that he could lose his land,

Sergio had decided to try again.

He failed again and was deported for a second time.

He's given up for now.

But migrants like him will keep on trying out of desperation.

Their chances of success are increasingly slim.