I. TO THE PEACE OF NICIAS (421 B.C.). TO THE DEATH OF PERICLES.--Wonderful as was the growth of Athens under Pericles, it is obvious that she stood exposed to two principal sources of danger. Her allies and dependants, the stay of that naval power in which her strength lay, were discontented with her spirit of domination and of extortion. The _Peloponnesian Alliance_, which was led by _Sparta_, the bulwark of the aristocratic interest, comprised, with the Dorian, most of the Aeolian states,--as Boeotia, Phocis, Locris, etc. Its military strength lay mainly in its heavy-armed infantry. Thus Sparta had the advantage of strong allies. The motive at the bottom of this alliance was what Thucydides tells was the real cause of the Peloponnesian war,--the jealousy which the growth of Athens excited in other states. This feeling really involved a conviction of the need of maintaining in Greece that which in modern times is called a "balance of power." When Greece was no longer one, as in the best days of the wars with Persia, but was divided into two opposite camps, watchful and jealous of one another, an occasion of conflict could not fail to arise. It was complained that Athens gave help to _Corcyra_ in a war with _Corinth_, its mother city, made war upon _Potidaea_ in Macedonia, a Corinthian colony, and also shut out _Megara_ from the harbors of Attica. The demands made by Sparta, which included the granting of independence to _Aegina_, were rejected. Attica was ravaged by Spartan troops, and the coast of Peloponnesus by the Athenian fleet (431 B.C.). This desolating warfare was kept up until a frightful pestilence broke out at Athens,--a plague having its origin in Egypt, and passing thence over Asia and the Greek islands. Two of the sons of Pericles died, and an accumulation of public burdens and private sorrows brought on his own death (Sept., 429).