For centuries, the dilemma facing Iran (and before it, Persia) has been guaranteeing national survival and autonomy in the face of stronger regional powers like Ottoman Turkey and the Russian Empire. Though always weaker than these larger empires, Iran survived for three reasons: geography, resources and diplomacy. Iran's size and mountainous terrain made military forays into the country difficult and dangerous. Iran also was able to field sufficient force to deter attacks while permitting occasional assertions of power. At the same time, Tehran engaged in clever diplomatic efforts, playing threatening powers off each other.
The intrusion of European imperial powers into the region compounded Iran's difficulties in the 19th century, along with the lodging of British power to Iran's west in Iraq and the Arabian Peninsula following the end of World War I. This coincided with a transformation of the global economy to an oil-based system. Then as now, the region was a major source of global oil. Where the British once had interests in the region, the emergence of oil as the foundation of industrial and military power made these interests urgent. Following World War II, the Americans and the Soviets became the outside powers with the ability and desire to influence the region, but Tehran's basic strategic reality persisted. Iran faced both regional and global threats that it had to deflect or align with. And because of oil, the global power could not lose interest while the regional powers did not have the option of losing interest.
Whether ruled by shah or ayatollah, Iran's strategy remained the same: deter by geography, protect with defensive forces, and engage in complex diplomatic maneuvers. But underneath this reality, another vision of Iran's role always lurked.