The US military has joined Israel in attacking more than 1,000 targets in Iran, killing many of its top officials, including Supreme Leader Ayatollah Ali Khamenei.
"Reuters" provides a look at the legality of the US strikes, which critics say exceed the president's authority and disregard international law.
What is Trump claiming?
President Donald Trump has offered a variety of goals and justifications. He said he believed Iran would strike first and that the attack was intended to eliminate immediate threats to the United States, its military bases abroad and its allies. He did not provide details, however, and some of his claims were not supported by US intelligence reports. Trump also suggested that Iran could have a nuclear weapon within a month, but provided no evidence, contradicting his June claim that the US military had "destroyed" the country. Iran's nuclear program.
Presidential authority to use military force
The attacks on Iran extend the limits of Trump's constitutional authority, legal experts say.
Under the U.S. Constitution, the president commands the armed forces and directs foreign relations, but only Congress has the authority to declare war.
Presidents of both parties have carried out military strikes without congressional approval when it was in the national interest, but at a lesser intensity in duration and scope than what would be considered war - a threshold that Trump may be testing.
Trump and Defense Secretary Pete Hegseth have both characterized the action as a war, with Hegseth calling it "the deadliest, most complex and most precise air operation in history." Trump said it could last five weeks or more and warned that there would be more casualties on the US side.
Congress has authorized major military operations, such as President George W. Bush's invasion of Afghanistan in 2001 and Iraq in 2003.
War Powers Resolution
The 1973 War Powers Resolution (WPR) acts as a check on presidential power.
According to it, the president can involve the military in an armed conflict only when Congress has declared war or provided specific authorization, or in response to an attack on US territory or its military. The law requires the president to report regularly to Congress, something the administration began doing on Monday.
In addition, the War Powers Resolution requires that unauthorized military action be terminated within 60 days, unless the deadline is extended.
There is a procedure in place for Congress to withdraw troops from a conflict, and members of both parties have said they plan to vote on such legislation this week.
Such a vote is highly unlikely to garner the two-thirds majority needed to override Trump's veto.
Legal experts have said that popular opposition could be the main obstacle to Trump's ability to proceed with the attacks.
What does international law say?
Legal experts have noted that many countries would consider the attacks unjustified under the UN Charter, which states that member states must refrain from the use of force or the threat of force against other states.
There are exceptions when force is authorized by the UN Security Council or is used in self-defense in response to an armed attack, but neither of these hypotheses applies in this case. There is also the concept of preemptive self-defense, which would allow the United States to attack Iran if it has evidence of an imminent, overwhelming attack.
The United States has veto power in the United Nations Security Council, which protects Washington.
Legal experts have said that violating international law still carries a price, and both the United Kingdom and Spain have limited the use of their bases in attacks, citing the lack of justification for the conflict.
Was the assassination of Khamenei legal?
Legal experts have explained that this is unclear.
Israel reportedly carried out the actual strike that killed Khamenei, and the United States offered intelligence and operational support.
In 1981, Republican President Ronald Reagan signed an Executive Order 12333, which prohibits anyone working for or acting on behalf of the U.S. government from participating in assassination. It also prohibits the U.S. intelligence community from participating in assassination.
The assassination of a leader, which may qualify as a peacetime assassination, can, however, be a legitimate military act during an armed conflict, legal experts have said.
In Khamenei's case, legality would depend in part on whether the United States was at war when he was killed and whether he was considered a military leader.