The Nixon Doctrine marked a turning point in American strategies of containment in the Persian Gulf. Whereas Lyndon Johnson had sought to balance Iran and Saudi Arabia as the “twin pillars” of the region during the British withdrawal “east of Suez,” between 1969 and 1972 Nixon gradually adopted a policy of Iranian primacy. Declining Anglo-American power does not provide an adequate explanation for this shift in U.S. Gulf policy. These constraints confronted both Johnson and Nixon, yet each president adopted quite distinct Gulf policies. Drawing on American, British, and Iranian sources, this article makes the case that the shift in U.S. Gulf policy from balancing under Johnson to Iranian primacy under Nixon reflected a change in American thinking about the shah of Iran, Muhammad Reza Pahlavi. This change in American thinking provided fertile ground for the shah's relentless efforts to secure Washington's backing for Iran's regional primacy throughout the 1970s.