The captain of the Hampshire grenadiers… has not been useless to the historian of the Roman Empire.

Gibbon is wryly acknowledging that his own military experience—specifically as a militia officer—proved unexpectedly valuable to his later work as a historian. The remark suggests that practical familiarity with discipline, logistics, command structures, and the everyday realities of soldiery can sharpen a historian’s understanding of ancient warfare and imperial administration. It also carries a characteristic Gibbonian irony: the seemingly modest or provincial role of a “captain of the Hampshire grenadiers” is contrasted with the grand subject of the Roman Empire, implying that large historical syntheses are often informed by small, concrete experiences. The line thus defends experiential knowledge as a supplement to book learning in historical writing.