Its all a question of propaganda.
When pope Urban called for the 1st crusade he defined the muslims as non-human heretics.
So the invading christian troops had their legitimation when they massacred nearly the whole population of Jerusalem.
They were the good ones and the others the bad ones.
In an elder speach Bush called the operation a "crusade" - history repeats itself...
Now Bush speeks about the Axis of Evil - Axis was the term for the bad guys in WW2 (while you have to consider that the later bad guy USSR was in these times one of the good guys

).
People are easy to manipulate - especially in a perfect media system like the Wag-the-Dog-country.
If you believe that you are fighting for a legitime cause, for the good side and for the glorious future of your people, you do things and ignore others, which you would not do under normal circumstances.
The problem is that always innocents have to pay the price: The civilians who loose their relatives in the battles and the soldiers who have to die because of such politicians.