History of the Peloponnesian War

The defensive alliance lasted only a little while; then the Lacedaemonians and the Athenians quarrelled and, with their respective allies, made war upon one another, and any of the rest of the Hellenes, if they chanced to be at variance, from now on resorted to one or the other. So that from the Persian invasion continually, to this present war, making peace at one time, at another time fighting with each other or with their own revolted allies, these two states prepared themselves well in matters of war, and became more experienced, taking their training amid actual dangers.