It would be fair to say that Iran has been the pariah of international politics for over forty years. The United States has never got over its fit of pique over one of its favourite regimes (which it, with enthusiastic British support, put in place in 1953) being overthrown in what was, at the start, a popular revolution against its an extremely unpopular and repressive regime. The Shah saw himself as a 'modernising' , aggressively secularising, autocrat, in the style of Kemal Ataturk, who, for a time, was a model for regimes in West Asia and North Africa, such as those of Nasser in Egypt and the Ba'ath Party in Syria and Iraq. However, as can be seen from the political direction of Erdogan's Turkey, Kemalism was never fully accepted even by its own people. And so it also proved in Iran (for details, see Roy Mottahedeh's classic work, 'The mantle of the prophet: religion and politics in Iran.').
However, the United States did get something right. Iran is much too significant as a state for it to be geopolitically sensible for it to be isolated. It is a populous country, whose people is, compared to its neighbours Afghanistan and Pakistan, well educated. Through its borders with Turkey and Iraq and its significant Kurdish population, it is inextricably involved in West Asia, although its distinctive language and culture (and branch of Islam) separate it from most of the region. It borders Azerbaijan and Afghanistan, who also have substantial Farsi speaking populations. It also has a turbulent border region with Pakistan. There is no other state which is on the political tectonic plate which divides the three geopolitical regions of Asia from each other. It is not surprising that the United States' principal political purpose in its relationship to Iran (to isolate it) has completely failed. Iran is inextricably involved in world politics, whether anybody else likes it or not.
It has to be emphasised that the United States' sole reason for opposing the Iranian regime is that it departed the orbit of American military/political alliances after 1979. It is emphatically not because of the undemocratic and theocratic nature of the regime. When it suited the United States it was prepared to use Iranian clerics to overthrow the secular and nationalist (and democratically elected) Mossadegh government in 1953, for the crime of presuming to nationalise the oil industry. Nor does Iran's support of terrorism have much to do with it. The United States has been very close to states such as Saudi Arabia and Pakistan who are not innocent in this regard. As Saudi has shown recently, it is prepared to murder its own citizens on foreign soil. (Those with long memories may remember the murder of Orlando Letelier in the United States by the Pinochet regime in the 1970s, as an example of the immunity which the United States extends to the most murderous of its allies). Nor even is Iran's (possible) acquisition of a nuclear weapons capability a plausible excuse. For years, the United States acquiesced in Pakistan and India possessing such a capability : both these states could directly threaten Iran.
It is possible that a lifting of sanctions and a re-engagement with Iran could encourage the 'liberals' and disadvantage the 'conservatives' within the Iranian regime. However, it should be for the Iranian people to determine their own destiny without outside interference (and, had they been permitted to do so in 1953, the situation would perhaps be much better). I have no intention of defending the Iranian regime, because it is in many ways indefensible (although not as indefensible as the Saudi regime ) ; I am merely pointing out that the nature of the regime has nothing to do with the fundamental rationale for the United States' ostracism of it.
At present, and for several years now, the conflicts in West Asia (in Palestine, Lebanon, Syria, Iraq and Yemen) have been made more perilous by the involvement of both Iran and of Saudi Arabia. In this context, the so-called 'Abrahamic accords', masterminded by the Trump administration, between Israel and various Arab states, have sharpened antagonisms in West Asia, as recent events in the Gaza Strip demonstrate, and have not contributed to the resolution of any conflicts. It makes absolutely no sense to engage with one, and not the other, of these powers. The imminent withdrawal of the United States (and NATO) from Afghanistan and the continuing instability in Iraq make a diplomatic rapprochement all the more urgent.