ISIL (Islamic State in Iraq and the Levant) has found that racing to bottom in the bad manners department can be a winning strategy. American intel analysts and Special Forces newly assigned to Iraq (many had been there before) quickly noted the ISIL was deliberately changing their tactics to maximize the ability to terrify Arab soldiers and police. Thus the frequent, self-publicized massacres of adult male opponents, especially soldiers, police and tribal militia. When captured, sometimes with promises of mercy, these men are often massacred. This is often done in gruesome ways like crucifixion or beheading. As with the mass shootings, these murders tend to be captured on video and most are uploaded to the Internet for all to see. Adding to this obvious willingness to kill ISIL also encourages a willingness to die. Thus ISIL attacks often feature lots of suicide bombers or gunmen sent into situations they are unlikely to survive. Against professional Western (or Arab) troops this sort of thing doesn’t terrorize as much as it makes it easier to kill the ISIL attackers. But since the main foe ISIL has been facing has been armed Arab amateurs (or poorly trained soldiers and police) this aggressive use of terror often works. Not just in individual battles but on a wider scale in which it encourages foes to run away even before ISIL actually attacks.
This type of terror is nothing new. It’s been used for thousands of years. One of the most notable examples of this was the Mongol destruction of Baghdad in 1258. As was the Mongol custom when they came to a defended city they demanded immediate surrender. If that did not happen the Mongols made it clear they would take the place no matter the cost and then burn it down and kill everyone (except for some with exceptional talents the Mongols needed) in the city. The Arab ruler of Baghdad refused to surrender. The Mongols took the place in two weeks and burned it down and killed the several hundred thousand people still there. It took Baghdad several centuries to completely rebuild and repopulate. The destruction of Baghdad reminded every ruler in the region what would happen if they did not promptly surrender. Fortunately the ruler of Egypt at that time had a good army and decent subordinate leaders and inflicted a rare battlefield defeat on the Mongols, who decided to go elsewhere.
Al Qaeda has always been about terror, but often on a smaller scale. Individuals who opposed them (clerics, competent police and army commanders or politicians) were assassinated in an often successful effort to reduce the effectiveness of their opposition. Mass killings were discouraged, especially bombings that killed lots of women and children. ISIL continues this policy but has increased the terror tactics to include groups of opposing gunmen. Thus even competent army or police commanders find themselves with high desertion rates and subordinates who are rethinking their loyalties.
The terror works both ways. The Americans found that you don't have to kill terrorist leaders to disable them. For example, when American troops drove al Qaeda and the Taliban out of Afghanistan in late 2001, they captured large quantities of records dealing with al Qaeda administration. This was revealing. The al Qaeda leadership was constantly being criticized by subordinates for stupid mistakes, while the leaders were constantly monitoring their people for stealing and malingering. The U.S. made sure a lot of this dirty linen was leaked, and eventually the media in Moslem nations began reporting on it. This paid off when al Qaeda in Iraq, despite vigorous objections within the organization, increased the use of attacks that killed lots of civilians. This played a major role in al Qaeda going from an admired, to a despised, organization within the Islamic world. American intelligence leaked captured documents that made clear how uncaring the terrorist leaders were about these civilian deaths. Many ISIL leaders were present during this defeat and continue to go easy on the dead women and children.
ISIL talks about the evils of corruption but given the prevalence of corruption everywhere in the Middle East, it is unlikely they have completely eliminated it from their ranks. This, and the fact that American smart bombs and missiles are now once more going after ISIL leaders and other key personnel, is giving ISIL something to be terrified about. Thus the ISIL execution (by beheading) of the few Americans they have captured in an effort to force the U.S. to halt the air attacks.