40,000 Arabs on horseback invaded and took over Iran who had an army of a million (granted, they were in the middle of their over internal strife at the time) 1400 years ago and forced everyone to convert or die. There are currently 1.8 billion Moslems in the World, and somewhere between 100,000 to 200,000 Zoroastrians. You do the math.
There was no provocation for this, nor for its subsequent invasion of Rome from Anatolia all the way to its farthest reaches of North Africa.
Then again, like with any war, it was someone else's fault, because "We started it because we we're the bad guys" is something no one said ever.
Remember, Hitler starting WWI was purely 'defensive' as well, acoording to the Nazis, anyways.
ISIL was purely 'defensive' as well too.
As was 9/11.
As was the terrorist attack in Nice being reported right now.
Weird, I know.