Car bombs, seized US weapons, nearly an endless supply of foreign recruit. There are a lot of things which are cited as the reason for ISIS’s success on the ground in Iraq and Syria. Kurdish forces, who have been fighting ISIS repeatedly, see a different key for ISIS.
They are called Inghemasiyoun, ISIS shock troops who are described as both disciplined and fanatical, willing to fight to the death at a moment’s notice, and to blow themselves up rather than accept defeat. These fighters pledge “victory or martyrdom” upon their arrival at the battlefield, and they mean it.
ISIS tends to use a suicide bombing or two as a prelude to a full-scale offensive, putting the opposing troops into disarray, and then throwing these shock troops onto the front lines to wrap things up. They’ve gotten extremely good at it.
This is doubly a problem for their opponents on the ground from a morale perspective, as many of the other forces aren’t willing to die unconditionally at a moment’s notice, and knowing that the enemy is only weighs on their already sketchy morale.
This is the key to battles like the one for Ramadi, where Iraqi forces outnumbered ISIS by a great amounted, but routed in the face of suicide bombings and fanatical offensives. Iraqi officials may gripe about the troops disobeying orders by fleeing the battle, but most armies in the world simply don’t have the unconditional loyalty that ISIS commands, and expecting their troops to mirror ISIS in their disregard for their own safety is just not realistic.
One has to be careful of such evaluations. History has proved time and again that the 'fanatical' reputation of elite troops is both subjective and conditional, primarily dependent on both friendly and enemy capitulation to the propaganda about their perceived invincibility. The findings of the WW2 book 'Soldaten' recently called into question the perceived invincibility of the Nazi Waffen SS for example, who were not in actual hard evidence that much more effective on the field than regular German army units. What counted was the awe and fear they were able to spread amongst friend and foe alike.
The same is true throughout history, as we find that depending on the situation, in many WW2 battles the supposedly 'fanatical, crazy' Japanese surrendered in droves, Napoleon's feared Guard collapsed completely on more than one occasion, whilst going right back in time, Rome's praetorians performed seriously under par in battle on many occasions.
At the end of the day, 'elite' or 'fanatical' status should be seen primarily as a propaganda mechanism that helps to enforce the hierarchy of any oppressive and authoritarian military culture – regardless of its national origin. And as such its main objective is to coerce civilians into revering their military as a kind of sacrosanct and untouchable deity.