Yet in mid-July, Nusakambangan’s most hated inmate, the man who inspired the Bali bombers, Abu Bakar Bashir, was able to organise a room and invite 22  prisoners to a meeting. There, under the shadow of a sinister black flag, they pledged their allegiance to the Islamic State in Iraq and the Levant (ISIL),  also known as ISIS.

Bashir’s real influence is now debated in Indonesia, but he’s still one of the most charismatic jihadists in the country.

His bai’iat — or oath of allegiance — to  ISIL confirms to Australia and the West in general that the threat of terrorist violence against them in Indonesia is once again very much on the table.

“[The oath] is very much influential to all the Indonesian radical groups,” says Al Chaidar, a terrorism expert at the University of Malikussaleh in Aceh. “If Abu Bakar has pledged to ISIS it means ISIS is religiously legal to support.”

The last time Indonesia’s radical Muslims were religiously endorsed to fight jihad overseas, they received training as mujahideen in Afghanistan. They then returned to plan and execute a string of bombings including two Bali attacks which killed 222 people, including 92 Australians, as well as the atrocity against the Australian embassy in Jakarta.

Jihad has never died in Indonesia, but since 2009 it’s been fractured and inward-looking, and its primary target has been the “near enemy” — the local police force. Military training was “hiking in the mountains with 20 or 30 people with wooden weapons,” says analyst Taufik Andrie, the executive director of the Institute for International Peacebuilding.

Now the emergence of ISIL, its brutal leader Abu Bakr al-Baghdadi and its grandiose claims of an “Islamic State” or caliphate for all Muslims, has made that look like kids’ stuff.

It has provided a proving ground, a battlefield, and a powerful rallying cry to which young Indonesian men are flocking.

Authorities have no idea how many Indonesians have already gone to war — estimates range between 50 and 500. But thousands have sworn their allegiance in oath-taking ceremonies across the archipelago in Poso, Bekasi, Bima, Solo, Surabaya and Malang.

The fear is that, as the Afghan alumni did in the 1990s, these fighters will return to Indonesia having earned what Taufik calls their “proper military wings”, and then they’ll get organised.

“The danger lies maybe three to four years ahead, when people come back from Syria with increased weapons expertise, greater international context, deeper ideological commitment and combat experience,” says Sidney Jones of the Institute for Policy Analysis of Conflict. “They’ll take the recruits who are so easy to come by in Indonesia and fashion them into a lethal force.”

Taufik says these people are likely to slot back into the existing jihadist infrastructure. The local leader of Jemaah Islamiyah in Solo has told Taufik he’s hoping for at least 50 or 100 military trainers in two or three years time.

It’s likely, by then, that the target will once again be the “far enemy” — the West and its interests in Indonesia: businesses, hotels, tourists.

Nasir Abbas, an Afghan alumni who returned to a senior position in Jemaah Islamiyah, has confirmed that: “If America and its allies, including Australia, attack ISIS in Syria, then the ‘foreign enemy’ will be probably the next target.”

The commencement of American military operations in northern Iraq signalled this watershed moment, Nasir Abbas said, and, “The longer it goes and the more the West gets involved, the more and more people will support it”.

The Australian Federal Police South-East Asia manager Commander Chris Sheehan said the AFP in Indonesia was “concerned about the risks”.

It is understood the AFP is working with other agencies, including the Indonesian National Police, to try to prevent people travelling to the war zone.

The danger is real, but there are also crucial differences between Afghanistan in the 1980s and '90s and Syria now.

For one thing, the Afghan fighters were chosen and sent by existing radical groups while the Syrian fighters are often self-funded freelancers with little institutional backing.

Jones says the Afghans were training to fight the Suharto regime while ISIS is more interested in fighting Shiite Muslims — not a traditional target in Indonesia. Many fighters are also expecting to buy a one-way ticket only, either because they’ll die or because judgment day will come.

Also unlike in the 1990s, when the Indonesian government and the Muslim mainstream paid no attention to the danger brewing in their midst, they are now well aware of the threat posed by returning jihadis.

Supporting ISIL has been outlawed in Indonesia, and on August 8 and 9, anti-terror police unit Detachment 88 arrested some of its key local leaders including Afif Abdul Majid. Mass Muslim organisations, Nahdlatul Ulama and Muhammadiyah have also decried the violence preached by ISIL.

In his State of the Union address on Friday, outgoing president Susilo Bambang Yudhoyono characterised the fight against ISIL as a battle to defend Indonesia’s plural values.

“If our founding fathers fought for independence until the last drop of blood, to our generation today it is about fighting for our Indonesian-ness. Because of this, the government strongly rejects the spread of the misguided concept of ISIS … it is dangerous for our character.”

ISIL’s declaration of a global caliphate under Baghdadi, which he says Muslims are obliged to join, has also split Indonesia’s radicals. There can be no better illustration than that two of Abu Bakar Bashir’s own sons have rejected his pledge and left the organisation he founded, Jamaah Anshorut Tauhid.

The state crackdown has made adherents wary of talking to the media, but Muhammad Fachry, who runs the radical pro-ISIL website al-Mustaqbal highlighted the with-us-or-against-us creed that has created so much controversy.

Every Muslim must support the Islamic State and “if he doesn’t, he will die as an infidel”, Fachry told Fairfax Media.

As for the mainstream political and religious rejection of the caliphate, Fachry said: “The prophet Muhammad predicted that there would be clerics who would say anything to please the evil regime.’’

How all this pans out in Indonesia is, according to Sidney Jones, “a guessing game”. She is sure, though, that those who are going away to fight “will come back more dangerous than they left”.

They will re-enter a society already accustomed to accommodating returning religious warriors.

As Taufik Andrie says: “Every battle zone is a room to birth the new generation of jihad”.