Thucydides identifies what he regarded as the deeper cause of the Peloponnesian War: the growing power of Athens and the fear this created in Sparta. Rather than treating individual disputes as sufficient explanations, he points to a changing balance of power between the two leading Greek states as the underlying source of conflict.

The passage suggests that shifts in relative power can create insecurity even before either side deliberately seeks war. A rising power may become more assertive, while an established power may increasingly regard that growth as a threat. The resulting suspicion and strategic pressure can make disputes harder to contain.

In modern international relations, this idea is often associated with the “Thucydides Trap”, a term developed much later to describe tensions between rising and established powers. The quotation itself concerns the particular circumstances of Athens and Sparta rather than establishing a universal rule that such rivalries must end in war.