The terrorist army of Islamic State has missiles, tanks and bombs, but the potent weapon that allows a relatively small force to keep much of Iraq and Syria in its grip is bloodlust - in the form of beheadings, crucifixions and mass executions.

An army estimated at 25,000 - roughly the size of one U.S. division - is at war with Syria, has a large city near the Turkish border under siege and is believed to have set its sights on Baghdad. It also controls cities, roads and territory nearly the size of Great Britain, some 81,000 square miles, according to The National Counterterrorism Center.

Experts say the key to Islamic State's ability to project such power with relatively small numbers is the sheer brutality it uses to strike fear in the hearts of enemies. The beheadings of U.S. and British journalists and aid workers have shocked the west, but in northern Iraq and Syria, entire villages have come to fear the same fate, meted out without warnings or slickly produced videos.


“Their ferocity is the thing,” Shaul Gabbay, a professor at the University of Denver and analyst on Muslim world conditions, said to “The more fierce they are, the more successful they become because it strikes fear among not only the region, but the world.”

Even if the higher range of estimates of the size of the Islamic State army are true, it is stretched thin across the region. As it holds onto villages and cities already seized as part of its so-called caliphate, Islamic State is on the brink of seizing Kobani in northern Syria. Some 500 miles to the southeast, the militants have overrun a series of small villages and suburbs near Baghdad and now are reportedly just eight miles away from the Iraqi capital's international airport. In addition, the extremist group has been fighting Kurdish Peshmerga forces in northern Iraq and has major cities, including Mosul, under its control.

U.S.-led airstrikes and an Iraqi army that dwarfs Islamic State have provided scant deterrence.

It's not just Islamic State's embrace of savagery that makes it so effective. The fact that the terrorist army publicizes its brutality ensures that no one misses it. In June, an entire division of the U.S.-trained Iraqi army dropped their weapons, shed their uniforms and fled from the bloodthirsty marauders. They had heard of the beheadings that have become a trademark of Islamic State.

“Not only do they commit these horrific acts, they document it,” said Scott Stewart, vice president of tactical analysis for geopolitical intelligence and advisory firm Stratfor.

Stewart said Islamic State posts its horrors to social media, tweeting pictures of fighters hoisting severed heads and releasing video showing kneeling prisoners gunned down without mercy to strike fear in enemies and to recruit jihadists from among Iraq's alienated Sunni Muslim population.

“That’s been a big part of their growth, especially in Iraq and the Sunni population, “A lot of these people lost everything after the U.S. occupation and [former President Nouri al-] Maliki reneged on everything he promised the Sunnis when he was in office,” Stewart said.

While Islamic State has captured heavy equipment ranging from tanks to fighter planes, it still lacks the full trappings of a traditional army. That, according to Stewart, works in its favor, too. When fighting intensified at Kobani in recent days, reinforcements raced up from Iraq in nondescript Toyotas that were impossible for U.S. bombers to target.

“They can cover territory very quickly and get to one point from another with the light trucks that they use,” Stewart said.

Islamic State and all terrorist groups by definition have another tactical advantage over the U.S. and coalition forces, according toRyan Mauro, a national security analyst from the Clarion Project. They embrace death and revel in causing civilian casualties.

"A small number of martyrdom-seeking terrorists can do more damage than a larger number of militants who want to live and avoid civilian casualties," Mauro said. "The U.S. may avoid an airstrike out of concern for civilian casualties and Iraqi forces may choose retreat over death, but jihadists do not make such calculations. Because they seek death, their debate is only about which way of dying will be most fruitful."

Perry Chiaramonte is a reporter for Follow him on Twitter at @perrych