I've covered the rise of armed movements since 2001 in Pakistan, Afghanistan and today in Iraq. One of the reasons people tell me they have welcomed groups like the Pakistani Taliban and the Islamic State is that they bring with them an Islamic justice system that governs everything else.
"Swift justice" was one way a tribal elder in the mountains of northwest Pakistan described it to me.
Swift justice through Islamic law, or sharia. You begin to understand why this is such a powerful idea in Islam when you consider that it is derived from the Quran, the holy book that Muslims believe is the unchanged word of God, and from the Sunnah, the ways of the Prophet Muhammad.
However, how sharia is applied is often decided by whomever is dispensing justice and how they interpret the Quran and the Sunnah.
By telephone Al Jazeera spoke to a young man in Anbar province, western Iraq. His account of the Islamic State, and its justice, is worth repeating in full.
"When the Islamic State came we welcomed them. But now we want the army to come. The Islamic State raided the house of a former member of parliament, Wisam Aker al-Rawi.
"She was in her house in Rawa. The IS soldier tried to take her. Her family, some people from the town and the mosque elders managed to stop them, But still the IS took her two brothers and confiscated 3kg of gold and a large amount of money.
"A day later they freed them but only after they flogged them. They kept the gold and money. They have arrested and flogged others and they do this daily. They have set up a sharia court and they flog people without mercy."
The Ottoman caliphate, which came to an end in 1924, reconciled sharia with secular politics to formulate a justice system that reflected both.
That system, developed over hundreds of years, has been deliberately forgotten by armed groups such as the Islamic State.
They interpret sharia in its most literal sense, without heed to Islamic justice systems from the past that had proved popular and just.
Across pro-Islamic State social media over the last few years we've seen examples, brutal, horrific examples of that literal use, including beheading and hangings.
In many cases this type of justice has horrified not just the locals, but Islamic scholars and academics who have spoken out. In Pakistan, the Pakistan Taliban found their interpretation of Islamic law eventually fall foul of the public and "swift justice" for many became cruel justice.
The Islamic State group has a similar challenge in Iraq. Winning over the local population will be key if they are to be accepted. However many Iraqis who initially welcomed the group are now wondering whether it is their freedoms it fights for, or a divine justice system that only they are allowed to dispense.