Thucydides was in Athens when the Peloponnesian war broke out in 431 B.C. Soon after the war began, Thucydides perceived that it would be a conflict on a scale without precedent and he would become its historian. He ... began writing the History of the Peloponnesian War to which he devoted most of his life. Thucydides believed that there were patterns of behavior which emerged during the war, and that they could be repeated in the future. He was a mechanist in that he believed that when faced with similar problems, similar people would react in similar fashions. Believing that one may change immediate situations, the end products of large-scale enterprises are the result of mechanical forces present in nature and in man.