“they possess most gold and silver, by which war, like everything else, flourishes.”
Book VI, 6.34; "they have abundance of gold and silver, and these make war, like other things, go smoothly" ( trans. http://www.classicpersuasion.org/pw/thucydides/jthucbk6rv2.htm Benjamin Jowett)
History of the Peloponnesian War, Book VI