The United States’ justification for attacking Iran over alleged "direct threats” does not meet the threshold required under international law to wage war, legal experts said.

U.S. and Israeli forces launched a massive air campaign against Iran on Feb. 28, with Washington claiming it aimed to curb nuclear and missile threats from Tehran. Yet the war has also decapitated the country's government, and President Donald Trump is now demanding "unconditional surrender."

The White House laid out Washington's justification for the war during a news conference this week.

"This decision to launch this operation was based on a cumulative effect of various direct threats that Iran posed to the United States of America, and the president's feeling, based on fact, that Iran does pose (an) imminent and direct threat," Press Secretary Karoline Leavitt said Wednesday.

She went on to cite Iranian sponsorship of "terrorism," its ballistic missile program and its alleged efforts to "create nuclear weapons and nuclear bombs."

But Mary Ellen O'Connell, a professor at the University of Notre Dame, said the attack on Iran "had no justification under international law."

"The law is clear that international disputes are to be resolved using peaceful means – negotiation, mediation, the intervention of international organizations," said O'Connell, an expert in international law on the use of force and international legal theory.

The Trump administration has offered "vague mentions of imminent attacks by Iran and to prevent Iran from acquiring a nuclear weapon," while the U.N. Charter "requires, at the least, that evidence of a significant attack by Iran be underway," she said.

"No shred of such evidence has been provided. Nor is there any right whatsoever to start a war over a weapons program."

While Leavitt cited threats from missiles and militants, U.S. Secretary of State Marco Rubio offered a different justification for the war earlier in the week: fears that an Israeli attack would trigger reprisals against U.S. forces.

Brian Finucane, senior adviser for the International Crisis Group's U.S. Program, said there were several issues with Rubio's explanation, including that the Trump administration has since offered other rationales for the war.

"The U.S. probably could have prevented any Israeli attack on Iran by virtue of the leverage afforded by critical U.S. military support," said Finucane, who previously worked in the Office of the Legal Adviser at the U.S. Department of State.

The Iran war is not the only legally dubious military intervention by the Trump administration.

In early September, the United States began carrying out strikes on alleged drug-smuggling boats in the Caribbean and later the eastern Pacific – a campaign that has killed more than 150 people.

The U.S. government has yet to provide definitive evidence that the vessels it targets are involved in drug trafficking, and legal experts and rights groups say the strikes likely amount to extrajudicial killings.

Trump also ordered strikes on Iranian nuclear sites last year, and sent U.S. forces into Caracas in early January to kidnap Venezuelan President Nicolas Maduro, who is now on trial in the United States.

Finucane said Trump's Friday demand for "unconditional surrender" by Iran "further undercuts prior justifications for U.S. military action."

"The administration has not even bothered to argue that Operation Epic Fury complies with international law, but certainly statements like this make any such argument even less plausible," he said, referring to the Iran operation.