John Joseph Mearsheimer (December 14, 1947) is an American political scientist and international relations scholar, who belongs to the realist school of thought. He is the R. Wendell Harrison Distinguished Service Professor at the University of Chicago. He has been described as the most influential realist of his generation. Mearsheimer is best known for developing the theory of offensive realism, which describes the interaction between great powers as being primarily driven by the rational desire to achieve regional hegemony in an anarchic international system. He was a vocal opponent of the Iraq War in 2003 and was almost alone in opposing Ukraine's decision to give up its nuclear weapons in 1994, predicting that it would invariably face Russian aggression without a nuclear deterrent. His most controversial views concern alleged influence by interest groups over US government actions in the Middle East which he wrote about in his 2007 book The Israel Lobby and U.S. Foreign Policy. In accordance with his theory, Mearsheimer believes that China's growing power will likely bring it into conflict with the United States.
The book describes the lobby as a "loose coalition of individuals and organizations who actively work to steer U.S. foreign policy in a pro-Israel direction". The book "focuses primarily on the lobby's influence on U.S. foreign policy and its negative effect on American interests". The authors also argue that "the lobby's impact has been unintentionally harmful to Israel as well".

Both Mearsheimer and Walt argue that although "the boundaries of the Israel lobby cannot be identified precisely", it "has a core consisting of organizations whose declared purpose is to encourage the U.S. government and the American public to provide material aid to Israel and to support its government's policies, as well as influential individuals for whom these goals are also a top priority". They note that "not every American with a favorable attitude to Israel is part of the lobby", and that although "the bulk of the lobby is comprised of Jewish Americans", there are many American Jews who are not part of the lobby, and the lobby also includes Christian Zionists. They also claim a drift of important groups in "the lobby" to the right, and overlap with the neoconservatives.