1945--1953: A Constricted Monarchy

This era started off with the Soviet withdrawal crisis, in which Soviet troops stationed in northern Iran did not withdraw by the date agreed upon by the Allied powers, demanding an oil concession. The US, seeking to counter Soviet influence in the Gulf region, put pressure on the USSR to withdraw. This, in addition to the granting of an oil concession by the prime minister (though it was not ratified by the Majles, the Iranian parliament), let to the Soviet withdrawal. (p. 44-48, The Persian Puzzle by Kenneth Pollack)

The US then gave limited support to Iran, loaning the government about $25 million in 1947, less than the Iranians wanted. US military assistance was similarly small; for example, the US provided a $10 million military grant in 1950 and expanded its advisory mission to Iran's army. This period was characterized by disappointment by Iranian officials in the US, who wanted more support. (p. 48-51, ibid., Pollack)

Foreshadowing the 1953 US-supported coup, in 1946 "the shah offered the United States very favorable oil concessions if Washington would help him regain absolute power, but the United States demurred." (p. 48-50, ibid., Pollack) This stance is consistent with the Truman administration's in 1952-3, where the US put pressure on Britain and Mohammend Mossadegh, then Iran's prime minister, to come to a peaceful resolution to the crisis. However, once Eisenhower came to power, the US tone shifted dramatically as the Soviet threat, manifested in Iran's communist Tudeh party, became large in the minds of US officials (in particular the Dulles brothers, then holding the positions of Secretary of State and CIA director), despite its relatively minor role in the country's politics. Hence the US supported monarchist forces in Iran seeking to overthrow Mossadegh, who had reduced the Shah to a figurehead, and succeeded (despite a number of bumps along the way, including a failed coup attempt). From this, the US also gained major oil concessions from Iran. (p. 53-71, ibid.; All the Shah's Men, Stephen Kinzer; ch. 5, Overthrow, Kinzer)

1954--1979: The Shah Polices the Gulf for the US

The US quickly gave funds, to the tune of about $70 million in 3 weeks, to the new government to "consolidate its hold and stabilize the country." This the regime did with brutal efficiency, launching a "massive crackdown" in which 2,100 were arrested through the end of 1953. (p. 73, ibid.) Wary of a return to the tepid US support in past years, "The shah decided that the only way to convince Washington to give him what he wanted [in terms of economics and military assistance] was to make himself a key ally of the United States in the Cold War with Russia." (p. 76, ibid.) This the regime also did, and acted as a proxy for US interests in the region throughout this period. By the early 1960s, "Iran ... boasted one of the largest [US Agency for International Development] missions in the world."

The Shah was also able to purchase all the latest high-tech military equipment he wanted; the US also assisted in training the SAVAK, the shah's intelligence service, and the Iranian military. However, the Shah insisted that US officials rely on him alone for information on the domestic situation of Iran. Hence US officials were caught off guard by the brewing disconent in Iran.

As protests againt the regime mounted in 1978, the Shah was paralyzed, unwilling to use a massive level of force which might crush the revolt. President Carter's national security advisor, Zbigniew Brezinski, called the Shah to tell him that the US would "stand by [the Shah] no matter what course of action became necessary." (p. 132, Pollack)

1979--1994: Relations Turn Sour

Instead, the Shah chose to flee the country, eventually arriving in the US, much to the ire of Iranians, who had suffered under his brutal regime for decades. This led to the storming of the US embassy, and a prolonged standoff between the two countries, including a failed rescue effort by the US military.

In 1980, Iraq invaded Iran, supported by both reigning superpowers, as well as the Gulf Arab states. The Iranians, however, were hoping to obtain arms and even detente with the US, and with American hostages in the hands of Iranian-allied militant groups, a deal was made; this resulted in the so-called Iran-Contra scandal.

In the following years, Iran made some efforts to ease tensions with the US, including unilateral releasing of hostages. However, any US response was nonexistent if not hostile. (ch. 11-13, Treacherous Alliance, Trita Parsi) The final straw for Iran was when the US failed to invite Iran to the 1991 Madrid conference on Israel-Palestine; seeing this a major slight to their ambitions for regional power, they organized a competing conference including Hamas and other militant Palistinian organizations.

The era closes with the US announcement that the US will attempt "dual containment" of both Iraq and Iran.

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I kind of lost steam towards the end there; I've omitted plenty of material, and this is more of an 'off-the-cuff' response. I can fill in any details if you'd like. I also truncated my commentary at 1994 due to the subreddit's rules.