Here, in contrast, there is no plausible argument that attacking Soleimani, personally—or Iran, more generally—was necessary to protect the United States from the threat that would be posed by Iraq. Its proxy efforts notwithstanding, Iran is a distinct sovereign entity that is not directly seeking to occupy Iraq or forcibly take over the Iraqi government.
There used to be a fairly clear consensus on the non-application of the AUMF to Iran until this most recent turn by the Trump administration. Senior Obama officials wrote in an Expert Backgrounder, “there is no viable argument that [the 2002 AUMF] authorizes force against Iran.” In his nomination for Defense Secretary in July 2019, then-Secretary of the Army Mark Esper testified that the 2002 AUMF could not authorize the use of military force against Iran. And in June 2019, the administration informed Congress that “the Administration has not, to date, interpreted either AUMF as authorizing military force against Iran, except as may be necessary to defend U.S. or partner forces engaged in counterterrorism operations or operations to establish a stable, democratic Iraq” (emphasis added to highlight the exception).
Assuming that exception is legally valid (a big assumption), it does not appear to be relevant to the Soleimani strike. First, if the strike were to defend the U.S. forces engaged in counterterrorism operations in Iraq, the administration would have presumably invoked the 2001 AUMF as well. Second, as Tess Bridgeman wrote, “justifications that senior U.S. officials such as the Secretary of State have given — such as Soleimani’s threat to U.S. diplomats and citizens in other parts of the region — have no connection to the U.S. counter ISIS operations.”