I was speaking with a customer at Lowe's one day, about 10 years ago. Gent was a retired Army Colonel.
He was in the (expletive for manure) in WW2, Korea and Vietnam, retired after 46 years in harness.
Spent his last 5 years of his career at the Pentagon in contingency planning.
Got to talking about Islamist terrorists and their support base among Islamic folks, figured the fanatics were about 10% and supporters were perhaps 20% of the world's Muslims.
His opinion, would have to kill 30% or more of these two populations to break their will to fight.