Militant Islamist fighters take part in a military parade along the streets of Syria’s northern Raqqah province June 30, 2014. (Reuters)

Last week, as the forces of the Islamic State crept within 40 miles of Irbil, fear settled over the urbane Kurdish capital. People had heard of the militants’ brutality — of the crucifixions, the beheadings, the mass killings. They were understandably frightened, Kurdish journalist Namo Abdulla told The Washington Post. Some began to flee. Others made for the mountains. The killers were coming.

In the last week, images of the Islamic State’s savagery have been inescapable. News exploded yesterday of an image of a young boy, the son of an Australian member of the Islamic State, hoisting a severed head beside his proud father.

“This image, perhaps even an iconic photograph … really one of the most disturbing, stomach-turning, grotesque photographs ever displayed,” Secretary of State John F. Kerry said Tuesday morning. “A 7-year-old child holding a severed head, with pride and the support and encouragement of the parent, with brothers there. This is utterly disgraceful and underscores the degree to which [the Islamic State] is so far beyond the pale with respect to any standard by which we judge even terrorist groups.”

During a news conference in Sydney after the naming of a new Iraqi prime minister, Secretary of State John Kerry expressed cautious optimism in the new leadership. (Reuters)

The glorification of extreme violence using social media is one of the defining aspects of the Islamic State. The Sunni militants wield savagery like a tool, analysts say. It’s neither extemporaneous nor undisciplined. It’s concerted. It’s tactical. It’s evil. And that’s the point.

“There’s a strategic reason behind the executions,” wrote the Washington Institute’s Aaron Zelin. “And the gruesome pictures posted online for all to see.”

The seeds of today’s brutality were perhaps sown long ago in a 2006 book called “The Management of Savagery,” wrote expert Lawrence Wright in the New Yorker. The book, written by a radical Islamist thinker named Abu Bakr Naji, details patterns of “abominable savagery” witnessed in both the Islamic State and its earlier incarnations. According to this English translation, it calls for an “administration of savagery” and a merciless campaign to polarize the population, attract adherents and establish a pure Sunni caliphate. “We must make this battle very violent, such that death is a heartbeat away, so that the two groups will realize that entering this battle will frequently lead to death,” the book says.

Long before the Islamic State, such was the vision of a savage killer named Abu Musab al-Zarqawi. The Jordanian, an early rival of Osama bin Laden, wanted war between the two major sects of Islam — Sunnis and Shiites. And “for his purposes,” wrote Wright, “there was no better venue than the fractured state of Iraq, which sits astride the Sunni-Shiite fault line.”

That agenda, however, clashed with the vision of bin Laden. He eventually brought Zarqawi under his leadership in 2004, giving birth to al-Qaeda in Iraq (AQI) — a precursor of the Islamic State. It clashed frequently with the al-Qaeda command over its propensity for brutality. A bullet was good enough for killing, said bin Laden’s lieutenant. Why decapitate?

“What they failed to grasp was that, for Zarqawi and his network, savagery — particularly when directed at other Muslims — was the whole point,” Wright wrote in the New Yorker. “The ideal of this movement, as its theorists saw it, was the establishment of a caliphate that would lead to the purification of the Muslim world.”

Observers, including British analyst Alastair Crooke, say the “The Management of Savagery” set out the very ideology that the Islamic State has now carried out. Indeed, one of the first steps the book suggests is a “plundering of resources,” which the Islamic State pursued with the same fervency of its violent acts.

But then, Crooke says, the book calls for “massacring the enemy and making him frightened.”

“The management of savagery is defined very succinctly as the management of savage chaos!” the book states. “The increase in savagery is not the worst thing that can happen now or in the previous decade or those before it. Rather, the most abominable of the levels of savagery is [still] less than stability under the order of unbelief.”

There can be no mercy, it says: “Our enemies will not be merciful to us if they seize us. Thus, it behooves us to make them think one thousand times before attacking us.”

The killings aren’t just meant to terrify, the book says, they’re meant to “polarize” the population. The violence would fan sectarian tensions, make sects chose sides and “drag the masses into battle.”

It’s unclear whether this has happened on a large scale. But a recent report by The Washington Post’s Greg Miller suggests that the Islamic State’s “merciless reputation” ignited rampant defections among Sunni members of Iraq’s security forces. What’s more, Miller found, jihadists from North Africa are abandoning their groups to join the Islamic State in Iraq and Syria.

“In sum, the beheadings and the violence practiced by [the Islamic State] are not whimsical, crazed fanaticism, but a very deliberate, considered strategy,” analyst Crooke wrote. “The seemingly random violence has a precise purpose: It’s aim is to strike huge fear; to break the psychology of a people — and according to reports this is exactly what [it] has succeeded in doing.”