This attributed to Sun Tzu and his book The Art of War. Actually James Clavell’s foreword in The Art of War http://www.scribd.com/doc/42222505/The-Art-Of-War states http://www.collegetermpapers.com/TermPapers/History_Other/Sun_Tzu_vs_The_Wisdom_of_the_Desert.shtml, “’the true object of war is peace.’” Therefore the quote is stated by James Clavell, but the true origin of Clavell's quotation is unclear. Nonetheless the essence of the quote, that a long war exhausts a state and therefore ultimately seeking peace is in the interest of the warring state, is true, as Sun Tzu in Chapter II Waging Wars says that "There is no instance of a country having benefited from prolonged warfare. It is only one who is thoroughly acquainted with the evils of war that can thoroughly understand the profitable way of carrying it on." This has been interpreted by Lionel Giles http://www.dutchjoens.info/SunTzu%20-%20Art%20of%20War.pdf as "Only one who knows the disastrous effects of a long war can realize the supreme importance of rapidity in bringing it to a close."
Dr. Hiroshi Hatanaka, President of Kobe College, Nishinomiya, Hyōgo, Japan is recorded as saying "the real objective of war is peace" in Pacific Stars and Stripes Ryukyu Edition, Tokyo, Japan (10 February 1949), Page 2, Column 2.
Misattributed