Wars have been underexamined as causal factors in American political history. The nation's freestanding hot wars seem to have generated at least four kinds of major effects: policy changes of lasting consequence, new issue regimes, durable changes in electoral alignments, and durable changes in party ideologies. Considered here are the War of 1812, the War with Mexico, the Civil War, World War I, and World War II.David R. Mayhew is Sterling Professor of Political Science at Yale University (david.mayhew@yale.edu). His publications include Party Loyalty among Congressmen; Congress: The Electoral Connection; “Congressional Elections: The Case of the Vanishing Marginals”; Placing Parties in American Politics; Divided We Govern; America's Congress: Actions in the Public Sphere, James Madison through Newt Gingrich; and Electoral Realignments: A Critique of an American Genre. The author thanks Alan Gerber, Matthew Green, Jacob Hacker, Sonam Henderson, Rogan Kersh, Philip Klinkner, Heinz Kohler, Joseph LaPalombara, Bruce Russett, Abbey Steele, and anonymous reviewers for their helpful comments.