Charting the journey of foreign recruits who join the terror group and the juggernaut advance of its battle-hardened leadership

The last leg of the journey to jihad starts on goat trails near the Turkish-Syria border and ends through one of many holes in a barbed wire fence stretching several hundred miles.

Just beyond, in some cases no more than several kilometres away, new recruits are received by extremist leaders waiting in boot camps in towns, villages and abandoned Syrian regime buildings.

There the indoctrination begins. Most who make the journey to Syria are already committed to the hardline worldview. And after 30 days of Islamic study, weapons training and frugal living, they are locked in to a lifestyle that few ever rescind.

"It was tough and uncompromising," said one man who spent time in a camp within sight of the Turkish border town of Kilis. "The trainers were Saudis and Yemenis. The trainees were from everywhere. I saw two blue-eyed Frenchmen and a Belgian and many from north Africa."

It has been this way in northern Syria since mid-2012 when foreign fighters from around the world started to descend on the country, about 18 months into the civil war.

The jihadis' entry was at first low key, almost polite. They would share planes and buses with tourists travelling to southern Turkey and pay local smugglers to get them to rendezvous points that had been relayed by those who had made it safely inside.

The trails get busier after dark. From June 2012, up to 40 men and boys were crossing each night from the Turkish town of Reyhanli alone.

"They were standing outside my property like cattle," a Turkish smuggler near the Syrian border town of Atmeh, who says he helped the jihadis cross for free, told the Guardian. "They would come every day for many months that year."

Within months, the extremists had organised into disciplined and structured cells, their growth enhanced by their benign approach to locals and their willingness, initially, to help Syrian rebel groups.

"We knew they were foxing, but what could we do?" said a leader of the town of al-Bab north-east of Aleppo, who was ousted along with his entire company of rebels earlier this year. From exile in Turkey, he added: "As the weeks continued they started to get more inflexible, more demanding, and then just ruthless. They gained power quicker than we thought."

In late April 2013, the Trojan horse act was realised, and what is now known as Islamic State (Isis) was born. The group had gathered such numbers and momentum that it was able to oust the local al-Qaida group.

Its rapid advance ever since has been abetted by a loose but effective military command structure, a logistics operation that depends heavily on co-opting locals for food and accommodation, and a fierceness in battle that has been developed by years of conflict in Syria and Iraq and which, for newer recruits, is almost instantly contagious.

"The only thing like it in terms of spread and potency was the Nazi party in the late 30s," a senior western diplomat said recently. "We are now dealing with a juggernaut."

At the head of Isis is Abu Bakr al-Baghdadi, the self-declared caliph. Baghdadi sits atop a military council that sets strategy and manages resources from the eastern edge of Aleppo through to north-western and central Iraq.

His commanders are almost all Iraqi, and many are alumni of the two US prisons in Iraq, Camp Bucca and Camp Cropper, which were closed in 2010. Beneath them are several dozen cadres who carry no rank, but have a vaunted reputation from their time on the battlefields of Iraq, both against US forces and Shia militias, and civilians.

These battle-hardened ideologues, according to one member of the organisation, give Isis a resilience that trickles down to the new foreign recruits, many of whom had not used a weapon before arriving in Syria.

"The intensity of their operations is extreme," the former Isis member said. "Sometimes they are fighting for weeks on end. They were boys then, but they are not now."

He said the intensity of the fighting had reduced the need for training camps. Some newcomers were thrown straight into battle, learning on the move.

Others were drafted into sourcing supplies, or working at the business end of Isis, smuggling oil and antiquities and what is left of Syria's state-owned enterprises.

"They don't have a central system for food or accommodation," he said. "They are all handled locally. But all ammunition requests are dealt with centrally.

"The leadership is central to most things. They have a very tight grip on Isis. No one is prepared to defy them."