From the use of suicide car bombs against enemy front lines to slaughtering prisoners of war to frighten opponents, Islamic State militants have proven brutally effective on the battlefield.
Here are five things that make the group a formidable force.
Many militant groups use suicide bombers, from Al-Qaeda to the Taliban. But none uses them with such frequency on the battlefield as Islamic State.
"They have an abundance of fighters willing to commit suicide," says Riad Kahwaji of the Institute for Near East and Gulf Military Analysis in Beirut.
IS fighters use suicide car bombs like guided missiles when they attack an enemy front line, especially in urban combat. They then follow up the massive explosion with human wave attacks to overrun enemy positions.
"It is like the [international coalition] planes launching smart missiles. But the difference here is that there is more precision and also the shock element is bigger," Kahwaji notes. "It implants fear in the hearts of their adversaries."
Where does the IS group get so many fighters ready to commit suicide?
Kahwaji says new recruits are carefully sorted by their minders according to their ideological commitment and psychological state and then used accordingly.
"It seems they have their own system where they evaluate these fighters and see who is more extreme and willing to commit suicide and they are prepared for their missions and used when the need comes," he says.
In addition to suicide bombings, Islamic State seeks to terrify its enemies by cultivating an image of extreme brutality.
The world has been shocked by the beheadings of hostages, but the militants' enemies on the battlefield are subject to another tactic -- the executions of prisoners of war in full view of their lines.
"They have slaughtered a number of their prisoners of war in front of the adversary just before the attack, also to implant fear," Kahwaji notes.
Empty bullet casings are seen at a base in the Raqqa countryside used by Kurdish fighters after Islamic State fighters said they took control of the area.
Those killed before battles include prisoners of war from the Syrian Army as well as from the Free Syrian Army, a rival secular-led Syrian opposition group.
At the same time, Islamic State has posted videos of prisoner executions on the Internet.
In June, it posted a video of hundreds of Iraqi Army soldiers being executed in Tikrit.
In August, it posted a video of at least 120 Syrian soldiers being executed in the province of Raqqa.
IS fighters also use fear against civilians. When they take areas belonging to non-Sunni communities, the militants routinely abduct and shoot men of military age and, in the case of the Yazidi community, abduct and sell the young women into sexual slavery.
Fear of such treatment motivates civilians to evacuate territories en masse as IS militants approach.
The IS force is highly mobile, relying on pickup trucks to deploy fighters in small groups and at high speed. This makes it difficult for the international coalition to inflict heavy damage on the group from the air and explains why Western officials say it will not be able to defeat the group from the air alone.
An image grab taken from a video uploaded on social networks in August shows young men dressed only in their underwear being marched along a road before allegedly being executed by Islamic State militants.
Analysts say that despite the fact the IS group has captured tanks from the Syrian and Iraqi armies, it does not regularly deploy these on the battlefield, where its enemies have air superiority.
"The Islamic State uses the tanks just for propaganda pictures. They don't use them on the battlefield because just one bomb from the air would destroy the tank," says Fabrice Balanche, an expert on Syria at the French research center Gremmo in Lyon.
He notes that IS also embeds itself deep in the civilian population in towns it occupies, such as Raqqa in Syria. That makes it hard to launch air strikes against them without risking civilian casualties, which would create new recruits for the extremists' cause.
"If you bomb houses, you will quickly have strong opposition from the civilians, who are going to say it is a new crusade against Islam and that the Americans are killing people like the Israelis in Gaza," Balanche says. "We could very quickly lose the psychological war against IS."
Islamic State has grown from some 10,000 fighters in June to anywhere between 20,000 and 30,000 on the back of its successes in sweeping across northern Iraq.
Most of the new recruits have come from other militant groups pledging allegiance to IS. But the extremist group is also able to attract thousands of jihadists from around the world, including from Europe.
"It's in the hundreds, it may well be in the thousands, and it certainly seems to be the biggest-ever flow of Muslim foreign fighters to a battlefield abroad in modern history," notes Shashank Joshi, an associate fellow at Royal United Services Institute in London.
Joshi notes that one reason many foreigners go is that Syria is on the very edge of Europe and easy to reach through Turkey. Other jihads in Afghanistan, Somalia, or Yemen were far less accessible.
But IS has also been successful in recruiting because it casts the war in Syria as an armageddon between the Sunni and Shi'ite sects of Islam, where the governments in Damascus and Baghdad are supported by the Sunni Arabs' ancient rival, Iran.
"That attracts [Sunni] Muslims in Europe who would otherwise be more immune to this but are catalyzed by the imagery they have seen for three years now of suffering, of mass murdering, of air strikes in Syria, and that is a really potent recruitment tool that Al-Qaeda never had to the same extent," Joshi notes.
By carving out a physical territory in Syria and Iraq, IS has also given the idea of jihad a second wind.
"Al-Qaeda always focused on long-distance terrorist attacks against a far-away enemy and on avoiding acquiring territory because it was easy to target," says Joshi. "The Islamic State's declaration of a caliphate and tactic of focusing on a near enemy and gaining territory was a refreshing idea after years of Al-Qaeda's approach."
In eastern Syria and Iraq, IS also poses as the Sunnis' champion against yet another enemy: the Kurds. It recruits by playing on local Arab fears that the Kurds, who took control of oil-rich Kirkuk in July, are seeking to build their own independent state on Arab land.
Finally, Islamic State has learned from Islamic militants' mistakes of the past.
One of those mistakes was to compete directly for power with Sunni tribes, something which enabled U.S. forces in Iraq to fund and arm a tribal rebellion -- the Sunni Awakening -- that successfully rolled the Islamists back in 2006.
Today, the Islamic State's leader, Abu Bakr al-Baghdadi, takes pains to forge links with tribal leaders instead. And he does so in a traditional Arab way.
"For instance, he asks the tribal chief to give his daughter to one of the emirs of IS, and like this you have a marriage between IS and the tribe," says Balanche. "He also asks the chief to give his son to fight with IS, so you have a blood link between the IS state and all the tribes in the area. If a tribe resists, he kills everybody."