That's not a conspiracy theory, but a verifiable fact: the systems of control overtly implemented after 9/11 justified by terrorism had largely been proposed (overtly), by many of the same people pushing them after 9/11, with other justifications (often, in the most recent prior attempt, the War on Drugs was the justification.)

There is a related conspiracy theory, though, that 9/11 was engineered (either a false-flag op or simply knowingly allowed to proceed) for the purpose of being used in that way.