Islamic State’s plan to blow up a commercial jet out of Sydney last month was “one of the most sophisticated plots that has ever been attempted on Australian soil,” police said last week. For reasons that aren’t yet clear, the terrorists aborted the attack, and police are confident the bomb would not have made it through airport security. Still, terrorists managed to acquire plastic explosives and the components needed to set it off—all shipped by airmail from Turkey. The Australian authorities discovered the plan only when, nearly two weeks later, they received a tip from a foreign intelligence service.

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