But as the war dragged on and both Athens and Sparta began to suffer economic and military attrition, Persia saw its chance to reassert its authority over the Greek world, particularly in Ionia, which had been a contested region since the Persian Wars. The ability of an exogenous actor, Persia, to manipulate and influence the war between the Greek city-states lends weight to the structural realist notion that weak states—here, Athens and Sparta relative to Persia—are always at the mercy of a stronger power's strategic calculations. Sparta, the paramount land power, saw Athenian expansion as an existential threat to its very survival; it found itself in a security dilemma with the other great power, each escalating its military preparedness and making war all but inevitable.