Beginning of the Roman Empire decline? edit

Who decided that 98 marks the beginning of the decline of the Roman Empire? This is a serious question. Trajan managed to conquer Dacia, and even if he loses some territories in the Middle East, at his death the Empire had its maximum size. At a minimum, the sentence should have a citation. --.mau. 19:16, 8 June 2016 (UTC) — Preceding unsigned comment added by .mau. (talkcontribs)