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The two nations have been at odds since the 1979 Islamic Revolution overthrew a government installed by Washington.
Trump pulled out of the 2015 Iran nuclear deal and reimposed strict economic sanctions.
Within days starting in late 2019, a militia rocket attack on an Iranian base killed an American contractor, which triggered an American missile attack, a protest that damaged the American embassy, and the drone strike that killed Iranian Gen. Qassem Soleimani.
Iran has retaliated for the U.S. drone strike that killed top Iranian Gen. Qassem Soleimani in Iraq Jan. 2.
Iranian officials said they launched more than a dozen ballistic missiles at two bases in Iraq where American soldiers are stationed. The Pentagon has yet to release casualty and damage reports. The Iranian missile attack came five days after Soleimani’s death as he was being driven away from the airport outside Baghdad.
Rarely have the United States and Iran engaged in such direct military action.
The two nations have been at odds since the 1979 Islamic Revolution overthrew a government installed and backed by Washington. But December 2019 stands out as the month when simmering tensions boiled over.
Looking back over the past year and a half, increasing sanctions on Iran by the United States played a role. But Iran also found itself freed up to focus on other fronts, as the Islamic State was defeated and Syria gained the upper hand in its civil war.
Here is the runway of events that led up to this crisis.
President Donald Trump ran on a promise to pull out of the 2015 multinational agreement that rachetted back Iran’s nuclear program. Iran complied with the arrangement, but in May 2018, Trump terminated U.S. participation, and reimposed economic sanctions that had been in place prior to the deal.
Under a policy labeled maximum pressure, the Trump administration fully blocked the sale of Iranian oil, and the country’s economy tumbled.
The same month, Secretary of State Mike Pompeo unveiled the emergency sale of precision guided weapons to Saudi Arabia and the United Arab Emirates, Iran’s foes in the Middle East.
In Iraq, a lone rocket, likely fired by an Iranian-backed militia, landed in Baghdad’s Green Zone, the most secure area in the city and the site of the American embassy. There were no injuries.
Four oil tankers –– two Saudi, one Emirati, and one Norwegian –– suffered minor damage from explosives placed below the waterline. The careful nature of the attack (no vessel was at risk of sinking) suggested a "state actor." American investigators pointed the finger at Iran.
Iran shot down an American drone, claiming it had entered Iranian airspace. Trump says he called off a reprisal attack, because it would have caused Iranian casualties, which would have been "disproportionate."
Iraqi militias launched more attacks on oil facilities, including a rocket attack on a Exxon-Mobil energy project in southern Iraq, and the targeting of two Saudi oil tankers.
In early July, British marines impounded an Iranian tanker off the coast of Gibraltar, saying it was illegally delivering fuel to Syria.
On July 19, Iran seized a British-flagged tanker near the Strait of Hormuz. Ultimately, both the Iranian and British vessels were released.
At about the same time, Trump announced that U.S. forces in the Gulf had downed an Iranian drone using electronic jamming, although Iran denied the loss.
On Sept. 14, 2019, a variety of missiles hit nearly 20 Saudi oil and gas facilities at Khurais and Abqaiq. Fighters in Yemen claimed responsibility, but all signs pointed to Iran. Secretary of State Pompeo said, "Amid all the calls for de-escalation, Iran has now launched an unprecedented attack on the world’s energy supply."
Dina Esfandiary at the Centre for Science and Security Studies at King’s College London sees that attack as pivotal.
"That was key to Iran and the region," Esfandiary said. "It showed Iran wasn’t afraid to target infrastructure, and Tehran managed to send a message –– block us from selling oil, then we’ll mess with your oil sales. It was a targeted and effective strike."
A lull followed the September oil attack. There were reports of Iran sending more short range rockets to its militia allies in Iraq. Iraqis citing government corruption, and Iranian and American interference in Iraqi affairs staged protests that turned deadly and brought down Iraq’s prime minister.
But active military confrontations were limited.
"It’s been largely quiet until late December," said Suzanne Maloney, senior fellow at the Brookings Institution Center for Middle East Policy in Washington. "In the interim, Pompeo and other U.S. officials seemed to articulate a red line for what would elicit a U.S. response — the death of U.S. personnel."
That came on Dec. 27, when a rocket attack on an Iranian base in Kirkuk killed an American contractor. The next day, American missiles struck Iranian-backed Iraqi militias in Syria and Iraq, causing many casualties.
On Dec. 31, Iraqi militia forces stormed the American embassy in Baghdad and set fires in perimeter buildings. With Iraqi government intervention, they left the next day.
On Jan. 2, the American drone killed Soleimani and several others, including an Iraqi militia commander, and Iranians traveling with Soleimani.
Trump and Pompeo said Soleimani’s death derailed imminent attacks that threatened hundreds of American lives.
Esfandiary said the pattern so far suggests otherwise.
"Tehran knows it would be crazy for it to do something large scale because it would lead to a conventional war with the United States, which it would inevitably lose," she said. "Rather, Iran’s modus operandi is to continue being a nuisance through small-scale attacks right and left, that it periodically escalates and intensifies."
But Middle East specialist Anthony Cordesman at the Center for Strategic and International Studies said the more active confrontations that started in May marked a tactical shift by Iran.
"Once ISIS was largely defeated, and Syria largely secure, the overall pace of events was pushing Iran to confront the United States over Iraq," Cordesman said.
In early December, a State Department official singled out Soleimani as a political threat. In the wake of public protests, Iraq’s prime minister had stepped down and negotiations were underway to replace him.
Assistant Secretary for Near Eastern Affairs David Schenker voiced a key American objection.
"Soleimani is in Baghdad working this issue," Schenker said at a Dec. 6 press briefing. "It seems to us that foreign terrorist leaders, or military leaders, should not be meeting with Iraqi political leaders to determine the next premier of Iraq."
Schenker said Iranian-backed Iraqi militia attacks on Iraqi bases were a sign that economic sanctions were working.
"They are clearly under fear and pressure, and they are lashing out," he said.
Asked how those attacks indicated progress, Schenker said, "Things sometimes get worse before they get better."
Update: This article has been updated to include the Iranian missile strike in Iraq.
Congressional Research Service, U.S.-Iran Conflict and Implications for U.S. Policy, Jan. 6, 2019
Congressional Research Service, Iraq: Issues in the 116th Congress, March 26, 2019
Reuters, Iran's Soleimani and Iraq's Muhandis killed in air strike: militia spokesman, Jan. 2, 2019
U.S. State Department, Assistant Secretary for Near Eastern Affairs David Schenker On Iraqi Global Magnitsky Designations, Dec. 6, 2019
New York Times, U.S. Strike in Iraq Kills Qassim Suleimani, Commander of Iranian Forces, Jan. 2, 2019
Email exchange, Anthony Cordesman, Arleigh A. Burke Chair in Strategy, Center for Strategic and International Studies, Jan. 6, 2019
Email exchange, Suzanne Maloney, deputy director - Foreign Policy, Brookings Institution, Jan. 6, 2019
Email exchange, Dina Esfandiary, research associate, Centre for Science and Security Studies, King’s College London, Jan. 6, 2019