As the five-year conflict in Syria grinds on, BBC Persian has found evidence that Iran is sending thousands of Afghan men to fight alongside Syrian government forces.
The men, who are mainly ethnic Hazaras, are recruited from impoverished and vulnerable migrant communities in Iran, and sent to join a multi-national Shia Muslim militia - in effect a "Foreign Legion" - that Iran has mobilised to support Syrian President Bashar al-Assad.
Many have since fled the battlefield and joined the refugee trail to Europe.
In a small town in Germany, we meet "Amir", an Afghan man in his early twenties.
He was born to refugee parents in Isfahan, Iran, and is now himself an asylum seeker in Europe.
Like most of the almost three million Afghans in Iran, he lived as a second-class citizen.
Without legal residency or identity documents, he found it hard to get an education or a job. Fear of arrest and deportation was a daily reality.
It was difficult to move around freely, get a driving licence or even buy a Sim card for his mobile phone.
But one day, Amir received an offer that changed everything.
"Some Afghans, who were close to Iran's Revolutionary Guards, approached me and my mates at the mosque," he said.
"They suggested we go to Syria to help defend the Shia holy shrines from Daesh," he added, using an acronym for the previous name of the jihadist group Islamic State (IS).
"They said we'd get passports and have an easy life afterwards. We'd be like Iranian citizens and could buy cars, houses..."
Amir was drafted into the Fatemioun Brigade, an all-Afghan unit commanded by Revolutionary Guards officers.
The training, he says, was very short - a fortnight of tactical movement and basic weapons handling - and conducted in strict secrecy.
"The night we entered the base at Qarchak, near Varamin in Tehran province, all our mobile phones were confiscated - and after two weeks' basic training, we were driven to the airport in buses with blacked-out windows," he said.
Despite having no passports, the Afghan recruits were flown directly to Syria on specially chartered jets.
"Everything was taken care of by the Revolutionary Guards," he said. "When we arrived, we saw the bullet holes and shell damage. It was a war zone. What I did and saw there affects me still. I can't sleep - I get angry for no reason."
Prof Scott Lucas of Birmingham University in the UK has been closely following Iranian involvement in Syria.
He says the first Afghan militias began to arrive in 2012.
"The Iranian Revolutionary Guards Corps decided that the Syrian military could not succeed on their own," he told the BBC. "The frontlines were too depleted and men were trying to avoid conscription."
The Iranians decided to set up a 50,000-strong National Defence Force to fight alongside the Syrian army.
With a shortage of willing fighters inside Syria, they began looking elsewhere - signing up Iranian Afghans, Lebanese, Iraqi and Pakistani Shia recruits.
'Blown to pieces'
As we travelled across Europe, we met many Afghan ex-fighters like Amir, and all told similar stories.
In the Moria migrant camp on the Greek island of Lesbos, a clearly traumatised teenage veteran describes how Afghan Fatemioun fighters were used as first-wave shock troops and were effectively disposable.
"Sometimes we had no supplies, no water, no bread - hungry and thirsty in the middle of the desert," he told us.
"We were light infantry and we'd have to walk 20-30km (12.5-19 miles) to face the enemy and then fight them.
"We would take ground at great cost and then have to hand it over to the Syrian soldiers. But they would usually lose it back to Daesh after a day or two.
"One night we were surrounded in an orchard. They fired an RPG [rocket-propelled grenade] towards us and I saw my mate blown to pieces, right in front of my eyes. After that, I hated war and started to fear war.
"For nights afterwards, I would picture my friend in my head and would think: 'My God, what happened to him!' I was really scared."
'Forced to fight'
At the port of Mytilene we found another group of young Afghan men. They all said they were ex-Fatemioun fighters.
One, who showed us his dog tags and de-mobilisation paperwork, explained how he had been effectively coerced into fighting in Syria.
"They took us to war by force," he says. "I wasn't happy with that but they said that because I was an Afghan who'd been arrested without identity papers, they'd either deport me to Afghanistan or send me to prison. I ended up being held in Asgar Abad detention camp before joining up."
He says he spent 12 months in Syria, as a tank driver and later a sniper, deployed across the country from Damascus to Palmyra. But when he finally got back to Iran, the Revolutionary Guards broke their promises.
"They gave me this small green identity document. It was just this 30-day temporary residency. I couldn't get a driving license with it - I couldn't even buy myself a Sim card!
"I complained and they said: 'You have to go back to do another tour of duty' - but I didn't want to. I ran away and here I am."
There are no official figures for how many Afghans Iran has sent to Syria - or how many have been killed there.
Human Rights Watch recently estimated as many as 10,000 Afghans may have been recruited by the Revolutionary Guards.
Iran's foreign ministry has denied any Afghans are being sent in an official capacity. The official narrative from Tehran is that they are all volunteers, off to defend holy sites of their own volition.
But every week in Iran there are more military-style funerals for fallen Fatemioun fighters.
And with a major government spring offensive around Aleppo in the offing, it seems Iran's Foreign Legion will be fighting - and dying - for President Assad for some time to come.
You can watch Fighting for Assad: Iran's Foreign Legion at the following times:
BBC News Channel (all times BST):
Friday 15 April 20:30
Saturday 16 April 00:30
Friday 15 April 14:30
Saturday 16 April 00:30
Sunday 17 April 05:30; 12:30; 18:30
Monday 18 April 08:00