Presidents George W. Bush and Barack Obama clearly possessed major differences in their approaches to international affairs. One area, however, in which their views and policies in important ways overlapped was how political ideologies were likely to shape U.S. security, particularly in the Middle East. Although both Bush and Obama early in their presidencies indicated that ideologies would play relatively unimportant roles in their Middle Eastern policies, developments soon pushed both administrations to reverse course. Bush attributed the September 11, 2001 terrorist attacks perpetrated by al Qaeda to this group's profound ideological differences with the United States, and the president made similar assessments about the root sources of America's enmity with Iraq, Iran, and Syria. These judgments led Bush to make regime change in these states a central component of his foreign policies. The most important-and costly-dimension of this objective was the 2003 decision to invade Iraq and the subsequent efforts to democratize it. To Bush, increased liberalization in the Middle East would significantly improve America's security. The result, as the president explained in his Second Inaugural Address, was that "America's vital interests and our deepest beliefs are now one. So it is the policy of the United States to seek and support the growth of democratic movements and institutions in every nation and culture, with the ultimate goal of ending tyranny in our world." Although Obama's pragmatic foreign policy inclinations ran even deeper than Bush's, he, too, was pulled in a more ideological direction by Middle Eastern developments. Massive popular protests that swept across much of the Arab world in 2011 resulted in the ouster of three authoritarian leaders, Zine al-Abidine Ben Ali in Tunisia, Hosni Mubarak in Egypt, and Muammar Qaddafi in Libya and in increased political repression in many others, including Bahrain, Iran, Saudi Arabia, Syria, and Yemen (Qaddafi's overthrow, like Hussein's, was largely due to the use of American military force). Obama called for liberalizing reforms-even in America's authoritarian allies-as the best way of quelling the protests and ultimately building more stable relations with the United States. Obama, just as with Bush, claimed a synergy between America's ideological and security interests when he laid out his administration's vision for U.S.-Middle Eastern relations in light of the 2011 "Arab Spring": " We must acknowledge that a strategy based solely upon the narrow pursuit of [material] interests.will only feed the suspicion [among the peoples of the Middle East] that has festered for years that the United States pursues our interests at their expense.[Thus] it will be the policy of the United States to promote reform across the region, and to support transitions to democracy.The United States of America was founded on the belief that people should govern themselves. And now we cannot hesitate to stand squarely on the side of those who are reaching for their rights, knowing that their success will bring about a world that is more peaceful, more stable, and more just." Were these U.S. leaders correct in their assessments of the importance of ideologies in international relations? Are, for example, large ideological differences between states a key source of hostilities? Are attempts to export particular ideological values effective strategies of conflict resolution? Despite the overwhelming importance of these issues, little consensus exists in either policymaking or academic circles about how or to what extent ideologies affect international relations. Most notably, proponents of the theoretical tradition known as realism, which is the dominant approach to the study of international politics, argue that the effects of ideologies in international relations pale in comparison to the effects of power. Thus leaders should neither believe that others' core security policies are a product of their ideological principles, nor dedicate significant resources to attempting to spread particular ideological beliefs and institutions abroad. My research in two books has attempted to add clarity to these critical, and highly controversial, issues and debates. My primary purpose is to provide a detailed framework for understanding to what extent ideologies matter in international relations, how they do so, and which foreign policies decision makers should implement to make best use of this information to advance their state's security interests. After examining the diplomatic history of key cases of the great powers' foreign policies from the French Revolution to the end of the Cold War, as well as the international relations of various Muslim-majority countries in the Middle East since the Cold War's end, I find that ideologies consistently had major effects on leaders' core international perceptions and policies. Most importantly, ideologies went a long way toward determining leaders' understandings of which states were likely to threaten and which states were likely to support their core domestic and international interests. Ideologies, in short, to a great extent determined leaders' perceptions of likely enemies and allies. I define an ideology as leaders' preferences for ordering the political world. Ideologies, in other words, are the specific, often idiosyncratic, political principles and goals that leaders both value most highly and use to legitimate their claim to rule. Any number of different social values and institutional objectives can comprise leaders' core ideological beliefs. Do politicians, for example, advocate for their country the creation or continuation of representative or authoritarian political institutions? Capitalist or socialist economies? Theocratic or secular values? The advancement of particular ethnicities against rival ethnic groups? Prominent ideologies include communism, fascism, liberalism, monarchism, and religious fundamentalism. Leaders' ideological beliefs not only have major implications for domestic politics, but international relations as well. Significant ideological differences dividing states' leaders frequently affect their foreign policies by shaping their understandings of the threats that they pose to one another's interests. Ideologies shape leaders' threat perceptions and consequent foreign policies by two main pathways. First, these variables play a key role in affecting how leaders' assess one another's international intentions. The greater the ideological differences dividing decision makers, the more likely they are to assume the worst about one another's objectives. Ideological enemies believe that conflict between them is in the long run inevitable. Even if ideological rivals in the present exhibit no hostility toward one another-or are even currently cooperating with each other-leaders will often assume that such amicability is temporary, and is bound to be replaced eventually with overt animosity. Important historical examples of ideological rivals' beliefs that conflict with one another is inevitable and the consequent adoption of policies that ensure this outcome are numerous. These views, for example, were central to the origins of the Second World War. Adolf Hitler of Nazi Germany repeatedly told the Wehrmacht leaders that the origins, objectives, and means of fighting the unavoidable war with the Soviet Union were rooted in the huge ideological differences between the two powers. Three months before Germany's attack on the Soviet Union, he told his generals that the "struggle [with the USSR] is one of ideologies and racial differences and will have to be conducted with unprecedented, unmerciful, and unrelenting harshness. . . . The commissars are the bearers of ideologies directly opposed to National Socialism. Therefore the commissars will be liquidated." In fact, the "main theme" of Hitler's reasoning for attacking the Soviet Union, according to the Chief of the Armed Forces High Command, Wilhelm Keitel, was to engage "the decisive battle between two ideologies." Intense suspicions of ideological rivals were also a defining attribute of the Cold War. Former Soviet Foreign Minister Maxim Litvinov stated in an interview in 1946 that the "root cause" of Soviet-American confrontation was "the ideological conception prevailing [in the Soviet Union] that conflict between the Communist and capitalist worlds is inevitable." Dwight Eisenhower similarly asserted early in his presidency that "anyone who doesn't recognize that the great struggle of our time is an ideological one.[is] not looking this question squarely in the face." "The central core of the great world problem is the aggressive intent of international communism." This was still the view of America's president over thirty years later. According to Ronald Reagan: "All of us need to be better informed about the unchanging realities of the Soviet system. We are in a long-term twilight struggle with an implacable foe of freedom." "We cannot assume that their ideology and purpose will change; this implies enduring competition." A second prominent way in which large ideological differences are likely to shape leaders' threat perceptions is by affecting their understandings of the dangers to their most important domestic interests, namely the preservation of their political power and the regime type they support. The greater the ideological differences dividing decision makers in different states, the greater their fears of domestic subversion are likely to be. Leaders will tend to worry that the success of ideological enemies abroad will be contagious, ultimately boosting the political fortunes of like-minded individuals at home, even to the point of revolution. Politicians will also tend to assume that international ideological rivals will provide aid to the latter's ideological allies throughout the system in an attempt to promote political change in targeted states. In these ways, international ideological competitions tend to be translated into domestic struggles for power and legitimacy. Fears of domestic subversion often have critical effects on leaders' international relations. British and French conservative leaders' fears of the spread of communism, for example, virtually precluded an alliance with the Soviet Union against Germany in the 1930s, despite the latter's massive rearmament policies and geopolitical expansion. French Prime Minister Leon Blum in a letter to the French ambassador to the Soviet Union, Robert Coulondre, explicitly referred to the ideological barriers to an alliance with the Soviet Union that blocked cooperation despite significant incentives to forge such a coalition: "A psychosis is being created according to which the Soviet entente leads to Communism; this fear tends to neutralize that which is inspired by the German threat and to paralyze cooperation among the pacific powers at the very time when this current ought to intensify." British Prime Minister Neville Chamberlain similarly explained in a private letter to his sister Ida in March 1939 why he continued to oppose an alliance with the USSR even at this late date: "I must confess to the most profound distrust of Russia.I distrust her motives which seem to me to have little connection with ideas of liberty and to be concerned only with getting every one else by the ears." Chamberlain in the spring of 1939 even threatened to resign "rather than sign [an] alliance with the Soviet." This is not to say that ideological differences create an absolute barrier to alliances. There are numerous historical examples of security cooperation among fierce ideological rivals. Nevertheless, it is fair to state that substantial ideological differences among states create powerful barriers to alignment that require particularly strong incentives to overcome. Ideological differences often create important delays in alliance formation, or lead to their premature dissolution due to ideologies' centrifugal effects. In my research, I have found that ideological differences and their perceived effects on states' international and domestic interests have been a central cause of conflict in a number of key cases, including in the aftermath of the French Revolution, the 1930s in Europe, the Cold War, and various conflicts between Western and Islamist groups. The opposite threat relationships often hold for states' leaders who are dedicated to similar ideological beliefs. Policymakers who share core ideological principles are likely both to interpret one another's international intentions in a mostly favorable light, and to view their domestic interests as interconnected. These dynamics have often resulted in significant cooperation among multiple ideological groups, including liberals, monarchists, fascists, religious fundamentalists, and even communists. For example, although the Sino-Soviet alliance of the 1950s was eventually replaced with enmity, it should not be forgotten the depth of the cooperation between the two communist powers that existed throughout the 1950s. Soviet economic and military aid to China was massive, and in important ways surpassed analogous support from America to its allies. Internal documents reveal that ideological affinity was a key motivating force for this impressive, in some ways unprecedented, aid. Soviet leader Josef Stalin, for example, told one of his associates that "if socialism is victorious in China and other countries follow the same road, we can consider the victory of socialism throughout the world to be guaranteed...Because of that, we must not spare any effort or resources in assisting the Chinese communists." Trust and as a result cooperation among liberal states has been the most impressive and enduring of any ideological group. No established liberal democracy has ever warred with another. This phenomenon is known in the international relations literature as the "democratic peace." Numerous liberal leaders have expressed high levels of trust about one another's international objectives. In a speech expressing his support for the European Union (EU), President Bill Clinton, for example, claimed that Americans "should develop ourselves to become a part of the group that shares our common [liberal] values.Of course, one day the EU will surpass the United States economically. But if we belong to the same group with our common values, who cares!" Britain's Prime Minister Tony Blair reciprocated these sentiments when he indicated that despite America's overwhelming power superiority in the post-Cold War world, liberal states need not fear American primacy. Hence his assertion in a July 2003 speech that "there is no more dangerous theory in international politics today than that we need to balance the power of America with other competitor powers." Every president of the United States in the post-Cold War period (Ronald Reagan, George H. W. Bush, Bill Clinton, George W. Bush, and Barack Obama) has expressed confidence in the accuracy of the predictions of democratic peace theory. One of the most important, and enduring, effects of ideologies on leaders' foreign policies is that these variables create powerful incentives for politicians to try to convert ideological rivals to their own legitimating principles. Because leaders tend to believe that hostilities with ideological enemies is in the long run unavoidable and cooperation with ideological allies likely, politicians will view regime exportation as a way of reducing the number of enemies in the system and increasing the number of allies. Fears of subversion to the principles of international ideological enemies add domestic incentives to work for the spread of one's principles abroad. Taken together, these beliefs explain why politicians of virtually all ideological beliefs-monarchical, liberal, fascist, communist, and religious fundamentalist-have attempted to export, including by force, their defining ideological principles and institutions. As Stalin explained to a Yugoslav communist leader, Milovan Djilas, in April 1945: "This war is not as in the past; whoever occupies a territory also imposes on it his own social system. Everyone imposes his own system as far as his army can reach. It cannot be otherwise." Stalin, though, was partly wrong in his assessment. Regime exportation was not new to the Second World War, but typical among ideological enemies both before and after this conflict. During the Wars of the French Revolution, for example, British leaders, in the words of Prime Minister William Pitt, had "no idea of any peace being secure, unless France returned to the monarchical system." Regime change thus remained a high priority for Britain throughout the 1790s. Within weeks of the outbreak of the First World War, British leaders in both private and public statements made the destruction of "Prussian militarism" in Germany a central war-fighting objective. As Prime Minister H. H. Asquith declared in a November 1914 speech: "We shall never sheathe the sword.until the military domination of Prussia [in German politics] is wholly and finally destroyed." Prominent Soviet officials at various points in the interwar period declared that international peace could "only be guaranteed by the victorious proletarian revolution in all capitalist countries," and that the end of war could result "only when the Soviet system has been adopted by all the countries of the world." Given these views, it is not surprising that efforts to help spread the communist revolution remained a priority to Soviet leaders in these years. In a 1962 report entitled the "Basic National Security Policy" of the United States, the Chairman of the Policy Planning Council at the State Department and a close advisor to President John F. Kennedy, Walt Rostow, asserted that because it was difficult "to envisage the survival of democratic American society as a beleaguered island in a totalitarian sea," it was a preeminent American interest to see rival regimes, including the USSR, "develop along lines broadly consistent with our own concepts of individual liberty and government based on consent." Two decades later in the 1983 top-secret National Security Decision Directive (NSDD)-75, Reagan officials stated that a critical objective of U.S. foreign policy was "to promote . . . the process of change in the Soviet Union toward a more pluralistic political and economic system in which the power of the privileged ruling elite is gradually reduced. The U.S. recognizes that Soviet aggressiveness has deep roots in the internal system." President George W. Bush made related augments in his Second Inaugural address: "For as long as whole regions of the world simmer in resentment and tyranny-prone to ideologies that feed hatred and excuse murder-violence will gather .and raise a mortal threat.We are led.to one conclusion: The survival of liberty in our land increasingly depends on the success of liberty in other lands. The best hope for peace in our world is the expansion of freedom in all the world." Exporting liberalism, the president concluded, "is the urgent requirement of our nation's security." In all these cases, states' key policymakers believed that ideological differences with other countries were the root cause of the dangers to both their international and domestic interests. A logical, almost inevitable, conclusion resulting from these beliefs was that the promotion of leaders' ideologies abroad became a central strategy of international conflict resolution and domestic stability. According to the political scientist John Owen, since 1500 states have used force on over 200 separate occasions to alter or preserve the ideological principles and institutions of another country. Recognizing that ideological enemies confront powerful incentives to export their legitimating principles says nothing, however, about the means by which leaders hope to accomplish this goal. One of the major problems with the Bush administration's efforts to promote liberalism in the Middle East is that this presidency's highly aggressive policies caused numerous people in the Muslim world - including liberals and reformers - to doubt America's commitment to liberalism. These actions included largely one-sided support of Israel in its conflict with the Palestinian people, sending suspected or confirmed terrorists to illiberal states (such as Syria) to be tortured to acquire information as part of America's controversial rendition program, and the indefinite detention and sometimes torture of accused combatants and terrorists at the Guantanamo Bay detainment camp in Cuba. Most damaging to America's reputation was the Iraq war. Using force to spread liberty seemed to many a contradiction that was difficult to resolve. America's inadvertent killing of civilians during the war - especially when accompanied by some deliberate barbarities, such as the much-publicized inhumane treatment of prisoners by U.S. forces at the Abu Ghraib prison in Iraq - was particularly damaging to America's liberal reputation. Invading and occupying a Muslim country also fueled many ordinary Muslims' nationalistic sentiments against the United States. The Bush administration's major mistakes in executing some of its core policies does not mean, however, that leaders should discount the importance of ideologies in international relations. History has repeatedly demonstrated the great significance of ideologies to leaders' threat perceptions and core international-security policies. Failure to incorporate ideological variables into states' security strategies is to ignore key dimensions of how the world works. One of the most important of my findings is that different ideological groups (i.e., different political parties or governing factions) from the same country at the same time frequently have vastly different international perceptions and policies. The more similar a group's ideological beliefs to foreigners, the less threatening their policies tend to be. Significantly, this tendency applies to Muslim-majority countries today. Liberalizers and reformers in Muslim-majority states are often much less hostile to the United States and its allies than other ideological groups in the same countries at the same time, despite often intense opposition to some of America's more provocative policies. Western leaders, as a result, have a security interest in trying to boost these groups' political power. In other words, although U.S. policymakers have made major mistakes in the past in attempting to spread liberalism in the Middle East, the logic underpinning this objective remains sound. The goal should be to correct past errors in this area without abandoning an emphasis on the importance of ideologies in international relations. *** * The argument presented in this essay is drawn from two books, The Ideological Origins of Great Power Politics, 1789-1989 (Cornell University Press, 2005) and The Clash of Ideologies: Middle Eastern Politics and American Security (Oxford University Press, 2012).