The observation that groups unify in the face of common threats is long-standing. At the level of the nation-state, this is called the “rally-'round-the-flag” phenomenon. In the case of the United States, the rally phenomenon is measured as a surge of public approval for the president when the nation is involved in an international crisis. Two hypotheses have been offered for why this surge of support occurs: (1) patriotism, as individuals respond to a threat by identifying with an in-group, in this case the nation and its president; and (2) opinion leadership, as the information environment changes because opposition leaders fall silent or support the president during a crisis and a portion of the public follows those elite partisan cues. Through three waves of scholarship, empirical evidence has cumulated about whether, when, why, and how much people rally in response to international crises (although much of the evidence is based on dynamics within the United States). The public’s reaction to a crisis is not automatic; sometimes public approval for the president goes up; other times the president’s approval ratings go down. A positive rally effect is associated with a variety of conditions, such as how prominently the event is reported, whether the White House actively frames the issue, the amount of criticism from opposition elites, and whether the country is at war or has recently concluded a war. The sizes of such rallies are variable, but on average, rallies in response to the deployment of force or international crises are small. Only wars (or other spectacular events like a large-scale terrorist attack) consistently provoke sizable rallies and these big events elicit an emotional reaction from citizens and a self-identification with the nation. Both hypotheses—patriotism and opinion leadership—are helpful in explaining why rallies occur and why they taper off over time. The “diversionary theory of war” or the “diversionary use of force” is, for obvious reasons, a companion literature to the scholarship on rally effects. The logic is simple: if the public rallies around its leader in the face of external threats, then the possibility exists that politicians will intentionally create crises or deploy military forces or start wars to enhance their own political fortunes. Scholars have spent much effort trying to locate patterns of diversionary behavior by American presidents and other world leaders with inconsistent and inconclusive results. But the cumulative findings from the rally-'round-the-flag scholarship show that leaders can’t expect much of a public rally from any but the most spectacular of international crises, such as full-scale war. These findings from the rally literature help to explain the lack of consistent empirical support for diversionary theory.