History of the Peloponnesian War
Thucydides
Thucydides. History of the Peloponnesian War. Crawley, Richard, translator. London and Toronto: J. M. Dent and Sons Ltd.; New York: E. P. Dutton and Co., 1914.
6. If any of the powers outside Peloponnese invade Peloponnesian territory, the parties contracting shall unite to repel them, on such terms as they may agree upon, as being most fair for the Peloponnesians.