Then it seems the only reason the continue is a lack of will on the part of our leaders (and outright sympathy from the left).
Well, it is a bit more complicated than that, because each level is more decentralized than the one before it. Conventional war gets out of the span of control of the Chief Executives policy circle and events devolve to the operational military leadership. Guerrilla or partisan war is conducted by units that may be called battalions or brigades but are loosely organized company-sized units pursuing more of a general goal on their own initiative than getting direct policy commands under strict control from a central authority (No matter what their nominal overarching authority may think). Terrorists act in cells, at a level of organization even lower than partisans, and there is no central authority whose destruction will stop them. Whether they are more more motivated by some major policy end (World Communism, annihilation of Israel, an independent Israel, making an occupying force's life unbearable, establishment of the Caliphate, eviction of the UK from Eire) or mere hatred and bigotry (Tutus/Hutsis, hatred of Jews, hatred of Hindus, racial hatred, whatever) really isn't material, the point is that the feelings run high enough to fuel something so decentralized that it comes close to the 'Leaderless resistance' concept. The only real way to defeat it is to both operate against the terrorists so successfully that they perceive their chance of success as quite remote, and at the same time incentivize cooperation so that the religious, ethnic, or tribal population from which the terrorists come discourages support of their objective, which means assimilation in one form or another is key to ending them.
Now the problem with assimilation is that the children of the successfully assimilated tend to get sucked back into the victim culture that their parents have worked their way out of, since the children never had that struggle and and earned the reward they enjoy, and so a certain percentage of them are vulnerable to that appeal.
The leadership's will to both destroy the active enemy and assimilate the peaceful population into your own socioeconomic paradigm is crucial to success. Europe and the US have done pretty well at the destroying part, despite some notable terrorist successes, but the whole multi-culti romantic idea has totally sabotaged the assimilation part.
