Wars spring from unseen and generally insignificant causes, the first outbreak being often but an explosion of anger.

The line distills a recurrent Thucydidean theme: the visible “spark” of war is rarely its true cause. Conflicts often arise from long-accumulating pressures—fear, rivalry, shifting power balances, economic interests, and misperceptions—that remain obscure to contemporaries or are minimized in public justifications. When war finally begins, it may look like a sudden eruption triggered by an insult or incident, but that outbreak is better understood as the release of pent-up tensions. The remark also warns historians and citizens against taking official casus belli at face value, urging attention to deeper structures and motives that make violence likely even before the first blow is struck.