(CNN) When the US government killing of top Iranian general Qasem Soleimani was first announced, officials from the Pentagon up to President Donald Trump were careful to make clear the strike was meant to head off an imminent attack on Americans.

That the strike, which took place last week without congressional approval or debate, should be conducted with the urgency of heading off an imminent threat is actually key to making it legal under US law. But Trump has subsequently made clear -- notably in comments Tuesday from the Oval Office -- that he was also motivated by retribution after the death of an American contractor at an Iraqi military base, possibly caused by Iranian-backed militias, or violent protests at the US Embassy in Baghdad.

Trump's Oval Office comments left out the idea of an imminent threat and echoed his arguments on a conservative radio show that the attack on Soleimani should have been carried out years ago, by either President Barack Obama or President George W. Bush, two men whose foreign policy Trump has repeatedly tried to undo

But Soleimani's killing could lead to war, even if it was an attempt to disrupt a terror threat. On Tuesday night, Iran responded with force, launching more than a dozen missiles at two Iraqi bases that hold US troops in what appears to be an act of retaliation.

In a statement, the Islamic Revolutionary Guards Corps, an elite wing of the Iranian military that is also known as the IRGC, said the attacks were "hard revenge" for the death of Soleimani.

Killing a terrorist as a security matter is one thing. Killing another country's general as part of a policy agenda is something else entirely. According to CNN's reporting , top officials who briefed Trump were surprised that he decided to strike Soleimani, who was visiting Iraq, because it was the most severe option presented to him.

Since those first public announcements about the strike, officials have refused to provide evidence of an imminent attack and instead have argued that Soleimani's previous actions meant he would continue to act the same way and that eliminating him was part of a larger strategy.

See below how the justification for killing Soleimani has evolved.

'Imminent threat'

Thursday, January 2 -- Pentagon statement announcing the strike says it was to deter "future Iranian attack plans"

"At the direction of the President, the US military has taken decisive defensive action to protect US personnel abroad by killing Qasem Soleimani," read the statement, which added that Soleimani "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region."

The statement also notes previous deadly actions by Soleimani and his Quds force, which it said was "responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more."

But the strike itself, according to the Pentagon, "was aimed at deterring future Iranian attack plans."

Friday, January 3 -- Secretary of State Mike Pompeo cites "imminent threats"

On Twitter, Pompeo said Trump had made the decision to eliminate Soleimani "in response to imminent threats to American lives."

During an interview with CNN, Pompeo referred to an active plot the strike was seeking to disrupt.

JUST WATCHED Pompeo's full interview on airstrike on Iranian general Replay More Videos ... MUST WATCH Pompeo's full interview on airstrike on Iranian general 09:29

"We want the world to understand that there was, in fact, an imminent attack taking place," Pompeo said. "The American people should know that this was an intelligence-based assessment that drove this."

Friday, January 3 -- Trump says Soleimani was caught plotting to kill Americans

While Trump mentions that Soleimani was plotting to kill Americans, most of his statement on Twitter focused on Soleimani's past actions.

"General Qasem Soleimani has killed or badly wounded thousands of Americans over an extended period of time," Trump said, adding that Soleimani "was plotting to kill many more...but got caught!"

JUST WATCHED Trump speaks after ordering strike on top Iran general Replay More Videos ... MUST WATCH Trump speaks after ordering strike on top Iran general 04:11

The President further justified the killing by saying it should have happened long ago.

"He should have been taken out many years ago!" Trump said.

Later, during a public announcement of the killing at Mar-a-Lago, the President repeated that there was an imminent plot the killing had disrupted.

"Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel, but we caught him in the act and terminated him," Trump said.

"We took action last night to stop a war. We did not take action to start a war."

Should have been killed years ago and 'retaliation'

Monday, January 6 -- Trump says Obama or Bush should have killed Soleimani

Without mentioning an imminent threat, Trump complained about his predecessors during a call-in interview with conservative radio host Rush Limbaugh and said they should have targeted Soleimani.

Trump specifically named Obama when he mentioned the international deal to give Iran access to oil markets in exchange for freezing its nuclear program.

"Well, this should have been done for the last 15 to 20 years," Trump said.

"Him in particular," he added, referring to Soleimani. "He was their real military leader. He's a terrorist. He was designated a terrorist by President Obama, and then Obama did nothing about it except give them $150 billion..."

And he named Bush when he mentioned Iran providing improvised bombs that killed Americans in Iraq during the Bush administration. Those might be legitimate complaints. But they are not imminent terror threats.

"President Bush should have taken him out," Trump said. "He is responsible for the IEDs."

Tuesday, January 7 -- Trump says Soleimani's past justifies his death and the strike was retaliation

Speaking to reporters in Oval Office alongside his meeting with the Greek Prime Minister, Trump was asked what he knew before the attack.

"I knew the past; his past was horrible. He was a terrorist," the President said. "He was so designated by President Obama, as you know. And he wasn't even supposed to be outside of his own country. He was. So right there."

Trump also said Soleimani was traveling with "the head of Hezbollah." The President appears to have been referring to Abu Mahdi al-Muhandis , an anti-US Iraqi militia leader who founded Kataib Hezbollah in Iraq and orchestrated attacks against US forces in the region.

"They weren't there to discuss a vacation," Trump said. "They weren't there to go to a nice resort someplace in Baghdad. They were there to discuss bad business, and we saved a lot of lives by terminating his life. A lot of lives saved."

Later in the same appearance, the President specifically said the strike was an act of retaliation.

"Don't forget, in our case it was retaliation because they were there first," Trump said. "They killed -- look, I don't have to talk about him for 18 to 20 years. He was a monster."

Planning attacks and past actions

Friday, January 3 -- US national security adviser Robert O'Brien says Soleimani was "planning attacks"

While Trump seemed to refer to a specific plot, his national security adviser said Soleimani had been actively planning attacks on American service members and diplomats.

"This strike was aimed at disrupting ongoing attacks that were being planned by Soleimani, and deterring future Iranian attacks, through their proxies or through the IRGC Quds Force directly, against Americans," O'Brien said. "As President Trump said today, this action was taken to stop a war, not to start a war."

Friday, January 3 -- Chairman of the Joint Chiefs of Staff says Soleimani was planning a "significant campaign of violence."

Speaking to reporters off camera, Chairman of the Joint Chiefs of Staff Gen. Mark Milley was asked if the threat was imminent, and he said, "Absolutely," but defined that imminence as days and maybe weeks. "We would be culpably negligent" if we didn't take action, he said.

Sunday, January 5 -- Pompeo says the specific time frame of Soleimani's plans is irrelevant

Rather than refer to a specific plot, Pompeo told Jake Tapper that the proof of Soleimani's danger is proved by his past actions.

"The American people have the evidence right in front of their eyes," he said. "We don't have to guess about what Soleimani was up to. We know what he did on December 27. He killed an American. And we know what he's done for years and years and years: killed hundreds of Americans."

JUST WATCHED Jake Tapper presses Secy. Pompeo on Iran strike Replay More Videos ... MUST WATCH Jake Tapper presses Secy. Pompeo on Iran strike 09:55

Tapper pushed Pompeo on what he meant when he'd referred to an "imminent" threat.

"If you're an American in the region, days and weeks, this is not something that's relevant," Pompeo said.

There may have been plenty of justification to target Soleimani. But that could require authorization by Congress, if the threat he posed was not imminent. Assassination has been illegal under US law since 1981. And while US officials have determined that Soleimani is a terrorist, he was also a ranking general in a sovereign government, so targeting him tempts war in a new way.

Intelligence was 'compelling' but not specific

Monday, January 6 -- Milley stands by intelligence

The top US general, Chairman of the Joint Chiefs Milley, said Soleimani's past means the US knew what he'd do in the future. Few people saw the intelligence, Milley said, but it convinced him.

"I'll stand by the intelligence I saw, that -- that was compelling, it was imminent and it was very, very clear in scale, scope."

But it was not extremely specific.

"Did it exactly say who, what, when, where? No. But he was planning, coordinating and synchronizing significant combat operations against US military forces in the region, and it was imminent."

Milley also made an interesting point that "in the world we deal in, we don't deal in certainties, we deal in probabilities."

"I think the probability is elimination of Soleimani disrupted for sure the plans that he was putting together," he said. "Does that mean it eliminated all threats? No."

Tuesday, January 7 -- Pompeo defines "imminence"

During a news conference at the State Department, Pompeo said Trump was given information from the intelligence community and other sources and that Soleimani's past actions were paired with the possibility of further attacks.

"If you're looking for imminence, you need look no further than the days that led up to the strike that was taken against Soleimani," Pompeo said. "And then you, in addition to that, have what we could clearly see were continuing efforts on behalf of this terrorist to build out a network of campaign activities that were going to lead potentially to the death of many more Americans. It's the right decision; we got it right."

Tuesday, January 7 -- Defense secretary says Soleimani's "time was due"

Arguing the intelligence was not razor-thin, but persuasive, Defense Secretary Mark Esper told CNN's Christiane Amanpour that Soleimani was caught "red-handed" in Baghdad meeting with another terrorist leader.

"I think the threat was being orchestrated by Soleimani," Esper said. "That's what the intelligence reported. That's what he was doing on the ground in both Baghdad and Damascus and elsewhere, and I think it was only a matter of days, certainly no more than weeks."

JUST WATCHED Esper: Evidence of Soleimani threat was 'persuasive' Replay More Videos ... MUST WATCH Esper: Evidence of Soleimani threat was 'persuasive' 14:54

But ultimately, according to Esper, Soleimani's fate was a long time coming:

"This is no innocent man. This is a terrorist leader of a terrorist organization, and his time was due."

This story has been updated with additional developments Tuesday.