The Middle East is undergoing an era of revolutionary change that is challenging the foreign policies of the United States and virtually all regional states. In this new environment, opportunities and challenges exist for a number of regional and extra-regional states to advance their national interests, while attempting to marginalize those of their rivals. Despite these changes, the Arab Spring and revolutions in countries such as Tunisia, Egypt, and Libya have not altered some of the more fundamental aspects of the Middle East regional situation. One of the most important rivalries defining the strategic landscape of the Middle East is between Iran and Saudi Arabia. The competition between these two states is long-standing, but it is especially important now. Political relationships that have endured for decades, such as the one between Iran and Syria, now seem to be in some danger, depending upon how current struggles play out. The stakes in this rivalry can thus become higher in an environment of revolutionary upheaval. In this monograph, Dr. W. Andrew Terrill considers an old rivalry as it transitions into a new environment. Saudi Arabia and Iran have been rivals since at least the 1979 Iranian Islamic Revolution. As Dr. Terrill points out, this competition has taken a variety of forms and was especially intense in the aftermath of the Iranian revolution. Under Iranian President Mohammad Khatami (1997-2005), the rivalry relaxed to some extent, but a permanent détente was not possible because of a backlash within the Iranian political system. The successor presidency of Mahmoud Ahmadinejad further damaged relations and the Saudi-Iranian relationship was dealt an especially serious setback over the Saudi-led intervention into Bahrain. Because the current Saudi-Iranian rivalry is taking place in a variety of countries of interest to the United States, an awareness of the motivations and issues associated with the rivalry is important to U.S. policymakers. Dr. Terrill clearly identifies the struggle as region-wide, encompassing countries as far apart as Egypt, Bahrain, Yemen, Lebanon, Syria, and especially Iraq, where the United States is preparing to withdraw almost all of its troops. He also notes that while U.S. interests often overlap with those of Saudi Arabia, such is not always the case. Saudi Arabia and the United States often work well together in seeking to contain Iranian influence, but Saudi Arabia also is an absolute monarchy opposed to Arab democracy or any democratic reform of the existing monarchical systems. The Strategic Studies Institute is pleased to offer this monograph as a contribution to the national security debate on this important subject, as our nation continues to grapple with a variety of problems associated with the future of the Middle East and the ongoing challenge of advancing U.S. interests in a time of Middle East turbulence. This analysis should be especially useful to U.S. strategic leaders and intelligence professionals as they seek to address the complicated interplay of factors related to regional security issues, the withdrawal of U.S. forces from Iraq, fighting terrorism, and providing for the support of local allies. This work may also benefit those seeking better understanding of long-range issues of Middle Eastern and global security. We hope this work will be of benefit to officers of all services, as well as other U.S. government officials involved in military and security assistance planning. Strategic Studies Institute.