  (3) Insofar as realists envision the world of states as anarchic, they likewise view security as a central issue. To attain security, states try to increase their power and engage in power-balancing for the purpose of deterring potential aggressors. Wars are fought to prevent competing nations from becoming militarily stronger. Thucydides, while distinguishing between the immediate and underlying causes of the Peloponnesian War, does not see its real cause in any of the particular events that immediately preceded its outbreak. He instead locates the cause of the war in the changing distribution of power between the two blocs of Greek city-states: the Delian League, under the leadership of Athens, and the Peloponnesian League, under the leadership of Sparta. According to him, the growth of Athenian power made the Spartans afraid for their security, and thus propelled them into war (1.23).