No plan survives first contact with the enemy.
https://quoteinvestigator.com/2021/05/04/no-plan/
Quote Investigator: In 1871 Helmuth von Moltke wrote an essay about military strategy that included a lengthy statement that was essentially equivalent to the concise adage.
No plan of operations extends with any certainty beyond the first encounter with the main enemy forces. Only the layman believes that in the course of a campaign he sees the consistent implementation of an original thought that has been considered in advance in every detail and retained to the end.
Over time Moltke’s statement was condensed to yield the currently popular adages.