Why did Thucydides identify Spartan fear of Athenian power as a major cause of the war?

In his History of the Peloponnesian War, the Greek historian Thucydides identified the growth of Athenian power and the resulting alarm, or fear, it inspired in Sparta as the fundamental cause that made the Peloponnesian War inevitable.12

Thucydides viewed this dynamic as a structural tension between an established power and a rising one. While specific incidents—such as disputes involving Corcyra and Potidaea, or the Megarian decrees—served as the immediate precipitating factors, he argued that these were symptoms of a deeper, underlying shift in the balance of power.21

By emphasizing this "fear" as the primary driver, Thucydides provided a framework for understanding how the imperialism and expansionist ambitions of Athens fundamentally threatened the security and status of the Spartan-led Peloponnesian League. This interpretation has become so influential in political science that it is often referred to as the "Thucydides Trap," describing a situation where the anxiety of a dominant power regarding a rising rival leads to conflict.132