These plotters were patient. They did not run stop signs or red lights. They paid their rent and utility bills. In short, until they struck with catastrophic force, they did nothing that would have allowed anyone to suspect them, let alone make an arrest. We talk in terms of a war against such acts.
War implies action.
This is different - and exactly how different nobody knows. There are millions of people around the world who wish the United States ill. Any two dozen of them could quietly get together and plan something similar, something they hope to carry out on a specific date some years hence. Such groups cannot be infiltrated - not even if we had a spy in every home. Letters must be signed and include a return address, but on request we'll omit the street address in the paper.