Orrin Judd has a good benchmark for knowing when we’re really winning the War on Terror:
You obviously can’t end terrorism–it’s been around for hundreds–if not thousands–of years and is a useful tactic for folks who can’t take on their enemies on the battlefield.
Nor can you wipe out Islamicism any more than winning the Civil War and WWII brought an end to white supremacist ideology and neo-Nazism. You can render it nugatory though, as those other pathologies are today. That will require the radical transformation of the Middle East towards liberal democratic protestant capitalism. There won’t be a V-I Day we can celebrate, where Islamicism officially surrenders, but it will be obvious to everyone that those militants who remain represent only a very marginal part of otherwise healthy and decent societies.
Perhaps this is an appropriate measure: we’ll have won when they get to the point where when they have an Oklahoma City Bombing of their own–and they will–their populaces hold such an action to be unacceptable and insist that their elected leaders hound the perpetrators and their fellow travelers the same way we did.