The American invasion of Iraq in 2003 created both opportunities and dangers for Iran. On the one hand, it toppled an enemy leader (Saddam Hussein) and liberated the fellow Shi’ites of Iraq to dominate a “democratic” government. On the other hand, it put the powerful military of Iran’s American enemy right on the country’s door-step.
An important role in developing the opportunities and confronting the dangers fell to General Qassim Suleimani. Suleimani occupied a powerful position in Iran’s government. The New York Times has described him as “an American vice president, chairman of the Joint Chiefs, and C.I.A. director rolled into one.” Suleimani worked to increase the power of Iraq’s Shi’ite majority, extend Iranian influence over Iraq and into Syria, and push the Americans to pull out. Some Americans and a good many Iraqis died in the ensuing violence. Both the Bush II and Obama administrations had thought about killing him. Both seem to have decided that killing Suleimani would not advance American strategic interests at those particular times. Clearly, President Trump and his closest advisors made a different decision. On 3 January 2020, an American drone fired two missiles that killed Suleimani and some of his myrmidons.
It is impossible at this early date to foresee the long-term consequences. Still, it is possible to suggest some factors that will influence events. First, the killing of Suleimani is unlikely to deepen the existing abyssal hostility between the two nations.
Second, domestic factors will push Iran to retaliate for the assassination. General Suleimani in the front rank of Iran’s leaders. Trying to deter the United States from weeding-out other leaders could push Iran’s hardliners toward action. The same is true of maintaining the regime’s legitimacy in the eyes of the public. You can’t get out big crowds every year to chant “Death to America!” in the streets and then go “Never mind” when you get slapped in public.
Third, there is a huge imbalance of power between the United States and Iran. American superiority in conventional weapons would probably preclude a real Iranian conventional attack on American forces. The recent missile strike in Iraq both hit a remote facility with few Americans present and was telegraphed hours in advance to allow the Americans to take cover. At the same time, President Trump claims to want to end the “endless wars” launched by the Bush II Administration. That desire should bar any attack on Iran by American ground forces.
This reality could shape the behavior of both sides. Iran can pursue an “asymmetrical” response. Iran could use allies like Lebanon’s Hezbollah or Iraqi Shi’ites to attack American forces or American interests. Those would not have to be limited to the Persian Gulf or even to the Middle East. One key factor might be how robust are American defense and intelligence resources for dealing with such “asymmetrical” threats.
On the other hand, American air power is there and ready to be used. For example, Iran firing missiles at American ships in or around the Persian Gulf would trigger air strikes. Those strikes might not be very restricted. They would inflict still greater public humiliation on the regime.
So, future headlines may be full of car bombs and “smart” bombs.
 At the same time, the Americans were occupying Afghanistan on Iran’s eastern border. You can see how Iranian leaders might get a little skittish.
 Max Fisher, “Is There a Chance Of a Wider War?” New York Times, 4 January 2020; Amanda Taub, “Will Strike Deter Attacks, Or Lead to Even More?” NYT, 5 January 2020.
 It is now impossible to know if the policy pursued by the Obama administration would have led to an actual improvement of Iranian-American relations or merely postponed the current confrontation.