(CNN) In the hours and days after Qasem Soleimani was killed in a US drone strike, his demise was described in various terms: President Donald Trump said he had been "terminated"; other US officials talked about a "targeted killing" and "lethal action."

But both the Iranian President and Iraq's Prime Minister said Soleimani's death was an "assassination" -- essentially a politically motivated murder.

The crux of the Trump administration's argument is that the threat posed by Soleimani's plans was "imminent" and that the US response was "defensive." A key requirement in order for a strike to be lawful under Article II of the US Constitution is that a threat must be imminent.

But targeted killings are permitted under international law in only very narrow circumstances, and some legal experts are skeptical that the White House's justification for the strike -- offered without evidence at the time of writing -- meets those standards.

The imminence test

Secretary of State Mike Pompeo told CNN that Soleimani was "actively plotting" actions which "would have put dozens if not hundreds of American lives at risk. We know it was imminent."

The Chairman of the Joint Chiefs of Staff, General Mark Milley, said on Friday that the threat was measured in "days, weeks."

"It all depends on what you call imminent," one official told CNN on Saturday, "but we believe he [Soleimani] was in the final stages" of ordering attacks when he visited Beirut and Damascus in the days before he was killed.

A senior State Department official, speaking on background, told reporters there was "overwhelming evidence that [Soleimani was] going to launch a military or terrorist attack" against US interests, and that arresting him was out of the question: "There's no way anybody was going to stop Qasem Soleimani in the places he was running around. And so you take lethal action against him."

A defensive strike

The State Department official also advanced the argument that the US had acted in self-defense. The official said the rocket that killed a US contractor in Iraq on December 29th "was the 11th attack in two months by Qasem Soleimani and his proxies that he orchestrates."

US officials concluded those attacks were carried out by Kataib Hezbollah, an Iran-backed Iraqi militia, whose leader was close to Soleimani and was killed in the same drone strike.

Self-defense is described in the United Nations charter as the right to respond to an actual and significant armed attack.

Agnes Callamard, UN Special Rapporteur on Extra-Judicial Executions, says targeted killings are permitted under international law but only under very strict conditions. The self-defense argument is only valid if there is evidence of an imminent armed attack, she told CNN, and it has to be proportionate to the threat.

Callamard, who carried out the UN's investigation into the killing of Saudi journalist Jamal Khashoggi, said the international law on self-defense was also evolving, calling it both "controversial" and "difficult." For every case in which the law is cited, "we must demand transparency and accountability," Callamard said.

Callamard told CNN that were an armed conflict between the US and Iran already underway, the laws of war would apply -- and Soleimani might therefore be a legitimate target. There would not need to be a formal declaration of war. But to most experts the evidence for an existing armed conflict between the two states is scant. On the contrary, they have been fighting a common enemy in the form of ISIS in places like Iraq.

And there has been no formal congressional authorization for a war with Iran.

The intelligence

Hina Shamsi, director of the National Security Project at the American Civil Liberties Union, says the Trump administration's justifications for killing Soleimani are, so far, unconvincing.

"The very limited circumstances in which use of force might be permitted under either domestic or international law quite simply haven't been met here," Shamsi told CNN.

At least not yet. President Trump hinted Sunday that some of the intelligence that led to the killing of Soleimani might be released. "We may discuss that," Trump said when questioned aboard Air Force One.

Oona Hathaway, editor at Just Security and a former special counsel at the US Defense Department, tweeted that "If there is more information, the President bears the responsibility of providing it. We should not have to guess."

A major problem with such extra-territorial targeted killings, Callamard said, "is the lack of oversight. Executives decide who may be killed outside due process, when it acts in self-defense, against whom and how."

A 'state actor'

Historically, the US has made a major distinction between the targeted killings of "terrorists"-- those who are not acting on behalf of a state -- and officers of a foreign government. But with the killing of Soleimani, scholars say, that distinction is much less clear.

The US government designated Soleimani as a terrorist in 2011, and the Islamic Revolutionary Guard Corps, including the Quds Force Soleimani led, was similarly labelled in April 2019.

The Obama administration made the case for, and vastly expanded, the targeted killings of terrorists. Perhaps the most notable instance of this was with the 2011 killing of American-born Anwar al-Awlaki, a senior figure in al Qaeda, who the Justice Department said in a leaked Justice Department memo posed "an imminent threat of violent attack against the United States."

The Trump administration has also used targeted killings -- with drone attacks in Somalia, Yemen, Syria and elsewhere, and most recently with the raid in Syria that killed ISIS leader Abu Bakr al Baghdadi

Karen Greenberg, director of the Center on National Security at Fordham University Law School, said the Obama administration used the 'imminence' argument in unprecedented and expansive ways in targeting al-Awlaki, and traces of that approach are evident now in the Trump administration's justifications of killing Soleimani.

But she says the Soleimani case is very different. "You can't take aim at an official inside a government and say that is not a war," she told CNN. The administration has "crossed a threshold, and the significance of that cannot be overstated."

Callamard agrees that Soleimani cannot be compared with Osama bin Laden or al Baghdadi, "because he was the representative of a state and that is a very different situation."

9/11 and mission creep

Besides international law, there are also questions about whether the operation was justified under US law, and specifically the 2002 Authorization for the Use of Military Force (AUMF) which addressed "the continuing threat posed by Iraq."

In a call with reporters on Friday, US National Security Adviser Robert O'Brien told reporters that the killing of Soleimani was "fully authorized" under the 2002 law.

Two years ago, the acting general counsel for the Defense Department, William Castle, said the AUMF had "always been understood to authorize the use of force for the related dual purposes of helping to establish a stable, democratic Iraq and of responding, including through the use of force, to terrorist threats emanating from Iraq."

If that interpretation is accepted, the operation to kill Soleimani might qualify. But the AUMF is now 18 years old and was designed with Saddam Hussein in mind -- not Iran. In fact, Iran is not mentioned once in the AUMF.

"What happened with the War on Terror is that we had mission creep beyond what the initial authorization was, but it does not extend to here," Greenberg said.

Greenberg believes the system is broken. "There has to be some kind of authorization for this war, and it can't be our continually-expanding-without-oversight drone policy that has guided us for the last ten years."

"This constant redefinition of terms to push aside legal restraints has come to the point we all feared it would. 'It's not an assassination, it's a targeted killing.' Or, 'It's not war, it's just the way we're doing business these days,'" Greenberg said.

Hina Shamsi at the American Civil Liberties Union sees the same risk: "There has been a years-long trend of US presidents claiming ever more expansive authority to kill and engage in conflicts that is not just legally incorrect, but dangerous from the perspective of peace and security."

A political furor

The criteria involved in Soleimani's killing are fast becoming a political issue in the US, one that is largely breaking down along party lines.

On Saturday, House Speaker Nancy Pelosi said the administration's notice to Congress prompted "serious and urgent questions about the timing, manner and justification of the administration's decision to engage in hostilities against Iran."

Democratic Sen. Chris Murphy asked on Twitter: "Did America just assassinate, without any congressional authorization, the second most powerful person in Iran, knowingly setting off a potential massive regional war?"

Senior officials have retorted that authorization was not required. "We did not feel the need to ask for authorization over basic rights of self-defense," one said.

While arguments will continue on whether the strike that killed Soleimani and several others was a legitimate act of self-defense or an assassination, Callamard told CNN that such actions are rewriting the rules -- and she believes a bigger issue needs to be addressed.

"What kind of institutions and rules will best protect people around the world, and do we think that kind of strike is conducive to an international rule of law?" she asked.

So was the death of the Iranian general arbitrarily ordered by an angry President, or was he legitimately killed to save dozens or even hundreds of American lives? Unless and until fuller details emerge of Soleimani's plans, the debate will likely intensify.