c. 1600, "attention to particulars," from French détail, from Old French detail "small piece or quantity," literally "a cutting in pieces," from detaillier "cut in pieces" (12c.), from de- "entirely" (see de-) + taillier "to cut in pieces" (see tailor).
French en détail "piece by piece, item by item" (as opposed to en gros), a commercial term used where we would today use retail, expanded the senses of the noun. Meaning "a minute account or narrative" is from 1690s; that of "an individual part, a particular" is from 1786. In fine arts, "a small, subordinate part," by 1823.
Military sense of "selection of an individual or body of troops for a particular service" is from 1708, from the notion of "distribution in detail of the daily orders first given in general," including assignment of specific duties.