It brings no joy to issue a public rebuttal against a valued colleague, but there are moments that demand no less. The occasion is the publication of an essay titled "The Israel Lobby and U.S. Foreign Policy," written by two professors, John Mearsheimer of the University of Chicago and Stephen Walt, the academic dean and my colleague at Harvard's Kennedy School of Government. In essence, their 82-page piece argues that U.S. policy in the Middle East has been hijacked by a pro-Israel "Lobby." "The core of the Lobby," they say, "is comprised of American Jews who make a significant effort...