Targeted assassination only works to eliminate two things: (technological) brains and charisma. Beyond that, other means work better.
From the very start, the former administration and military used a very effective technique, which could be called the “roach motel”. The problem was radicals dispersed around the world, impossible to fight, as such. We needed them to concentrate to fight our soldiers, instead of allowing them to conduct a terrorist war against our civilians at home.
The logic behind this is that very few terrorist wannabees have the chutzpah to leave their home country and travel to a foreign country to commit terrorist acts. Once these few were exterminated, replacements might take a generation or two to show up.
In any event, “roach motel” likely killed tens of thousands of the most dangerous of the terrorists, saving countless innocent lives.
The next step was to go against the financiers of terrorism. But they did miss a step here by not going after the charismatic leaders who instigated terrorism in the first place. These would have been covert killings, made to look either accidental or in some factional dispute.
Which leads into the next idea: getting red-on-red (terrorist against terrorist) conflicts going. If one faction wipes out another faction, it just saves us work.
This is a risky method. Recall the words of Frodo to Sam outside Cirith Ungol:
"But that is the spirit of Mordor, Sam; and it has spread to every corner of it. Orcs have always behaved like that, or so all tales say, when they are on their own. But you can't get much hope out of them fighting each other. They hate us far more, altogether and all the time. If those two had seen us, they would have dropped all their quarrel until we were dead."
Cheers!