There is nothing new about the use of terror as a weapon of war.
Alexander the Great used the tactics in his war with the against the Persians, way back in 332 BC. He was so furious at the way the fortified island city of Tyre held out against him, forcing him to waste much time and energy to defeat them, (it took months and he had to construct a causeway out to the island in order to capture the city) that he made an example of them. His idea was that everyone else should see what happened to people who resisted him.
He destroyed the city. Sold 30,000 into slavery and massacred 8,000 civilians. He had them crucified on the beach for everyone to see.
The message soon got round. Many came over to his side to escape his wrath and people were terrified to stand against him.
We can see the same tactics being used by ISIL. Nowadays they use social media to show mass beheadings, people locked in cars and blown up with rocket launchers, underwater film of people being drowned as they are slowly lowered into the water, people have their heads blown off with plastic explosive, being buried alive, thrown off high building, slowly crushed and people being burnt alive in cages. It is barbaric and gruesome, cruel, heartless and callous (and all in the supposed name of religion!) but, in the short term it is highly effective.
As with Alexander the Great it sends terror into the hearts of the enemy. Despite the fact that the Iraqi troops far outnumbered the ISIL troops ISIL have been able to take places like Palmira. The Iraqi troops were so terrified at the thought of what would happen to them if they were caught that they ran away.
In the long term it will lead to the utter annihilation of ISIL. The revulsion for their tactics will unite civilised people in opposing them. Even pacifist like myself are so repulsed by their primitive inhumanity that they want them, and their intolerant savage doctrine, eradicated. People will soon recognise that ISIL is not driven by religious conviction so much as megalomania and animalistic blood-lust. Their Jihadists are in it for the sex and excitement, the religious fervour is a sham.
Terror gives short-term advantage and a long-term heavy reckoning. ISIL’s days are numbered. The whole world has had enough of their evil.