O'Brien claimed in a conference call with reporters that Soleimani's killing in an SUV outside Baghdad International Airport on Thursday was authorized by the 2002 Authorization for the Use of Military Force (AUMF), a law permitting the US military to carry out attacks in undeclared war zones against al-Qaeda targets or organizations linked to al-Qaeda.
The adviser further noted that Soleimani had been traveling around the Middle East planning attacks against US military personnel and diplomats, including the December 27 attack on the K1 military base in Kirkuk that killed a US contractor. That attack set in motion a series of events leading to several thousand Iraqi protesters burning part of the US Embassy in Baghdad on December 31. The protesters, some of whom were from Popular Mobilization Forces militias linked to the Iraqi government, were angry that five US airstrikes on December 29 in response to the December 27 Kirkuk attack had killed 25 and injured 51 members of another PMF militia, Kata'ib Hezbollah.
The Iraqi government has denounced both the December 29 and January 2 airstrikes as violations of the country's sovereignty, as they were carried out without the permission of Baghdad. Lawmakers will convene an extraordinary session of the Iraqi parliament on Sunday in response to recent events, and are preparing to present legislation that would force US forces out of the country.
"We hope to have a good relationship with Iraq going forward," O'Brien said.
Earlier Friday, US Secretary of State Mike Pompeo told Fox News Soleimani was the "orchestrator, the primary motivator" of an "imminent attack" against the US.
Iranian Foreign Minister Mohammad Javad Zarif called Soleimani's killing "act of international terrorism."
"It was an extremely dangerous, foolish escalation ... He was the most effective force fighting against Islamic State and al Qaeda terrorists," Zarif told reporters Friday.
Soleimani had been the commander of the Quds Force, an elite unit of Iran's Islamic Revolutionary Guard Corps (IRGC). He helped lead the war effort against Daesh in Iraq and Syria and was considered one of the most politically powerful and popular men in Iran.
In April 2019, the US State Department declared the IRGC to be a terrorist organization, with the White House saying at the time the IRGC "actively participates in, finances, and promotes terrorism as a tool of statecraft."