Since the death of Iranian military commander Qasem Soleimani in a drone strike near Baghdad airport on Friday morning, U.S. officials have used careful language to describe an operation that they claim was justified.

The Pentagon on Friday called it a “defensive” action. President Trump said the strike was carried out to “stop a war.” State Department officials bristled at the use of the word “assassination” to describe the operation, saying, “Assassinations are not allowed under law."

But perhaps the most pertinent word used in these narratives, however, is one that some may have glossed over: imminent. It’s worth noting — especially because, in a legal context, it may not mean exactly what the average person thinks it does.

“An imminent threat is what you would need to justify taking an action in self-defense,” said Oona Hathaway, a professor of international law at Yale University and a former national security lawyer in the Defense Department’s Office of General Counsel.

The word has been repeatedly used by Trump and other members of his administration to describe alleged plots against U.S. personnel in the Middle East that they say Soleimani was facilitating and that the strike on him may have prevented.

Trump has spoken of “imminent and sinister attacks,” but the administration has provided little detail about the alleged plots or why the threat was deemed imminent. “We know it was imminent,” Secretary of State Mike Pompeo said during an interview with CNN on Friday, without offering any specifics.

Hathaway noted that the word was not used in the initial Pentagon statement announcing the strike against Soleimani. “It makes me wonder whether the lawyers realized that they had a problem on their hands,” she said.

As leader of the elite Quds Force, a branch of Iran’s Islamic Revolutionary Guard Corps, Soleimani had certainly played a role in significant plots against U.S. personnel in the past. There is little reason to believe that he was not involved in similar plots shortly before his death.

It is unclear, however, what specific evidence tied him to a new plot or how this evidence showed that neutralizing him would have foiled such a plot.

But the administration’s frequent use of “imminent” is probably not a coincidence. Both domestic and international law, in theory, place limits on a U.S. president’s ability to unilaterally engage in a military conflict with a foreign power: The 1973 War Powers Resolution, for example, states that a U.S. president requires congressional authorization to go to war.

Some analysts said the Trump administration appears unsure how to legally present its case for the killing of Soleimani.

National security adviser Robert O’Brien has told reporters that the 2002 Authorization for Use of Military Force Against Iraq Resolution allowed the killing of Soleimani, an argument that is almost certain to be disputed by lawyers and members of Congress because of the starkly different context of the Friday strike.

But by repeatedly describing an Iranian plot against U.S. personnel as “imminent,” the administration is making a potentially more persuasive argument: Its move against Soleimani was an act of self-defense, not aggression.

In both domestic and international law, that is an important distinction, and the idea of an imminent threat adds weight to it. “Imminence is relevant legally mainly as a way of constructing an argument of anticipatory self-defense,” said David Bosco, a professor at Indiana University’s Hamilton Lugar School of Global and International Studies.

Article II of the Constitution names the president as the commander in chief of the United States; it is generally accepted that this grants him or her the unilateral power to “repel sudden attack.” The Charter of the United Nations says there is an “inherent right of individual or collective self-defense if an armed attack occurs.”

The right to self-defense is widely understood to extend beyond attacks that have already occurred. “It is well-established that countries can respond preemptively if there is an imminent attack that gives little other option,” Bosco said.

However, it may not mean that the United States had evidence of a specific upcoming plot. The U.S. government generally takes a broad view of what imminent may mean. Though the Trump administration has been accused of taking a lax view of legal requirements, this broad definition predates it.

In an April 2016 speech that sought to explain the legal justification for the U.S. involvement in war against the Islamic State militant group, State Department legal adviser Brian Egan argued that there didn’t even need to be specific information about a threat for it to be considered imminent.

“The absence of specific evidence of where an attack will take place or of the precise nature of an attack does not preclude a conclusion that an armed attack is imminent for purposes of the exercise of the right of self-defense,” Egan said.

A November 2012 speech by Eric H. Holder Jr., the attorney general in the Obama administration, justified the killing of members of al-Qaeda by suggesting that the group’s past violence showed it could pose a continued threat.

“The Constitution does not require the president to delay action until some theoretical end stage of planning — when the precise time, place and manner of an attack become clear,” Holder said.

In this instance, Soleimani’s history of helping facilitate attacks on U.S. troops may have been enough to legally justify his killing. “This isn’t a question from a legal perspective of what he may have been planning next,” said Bobby Chesney, a law professor at the University of Texas. “It’s a question, first and foremost, of what he has done.”

Controversially, this broad definition is arguably not what most people would think of when they hear the words “imminent attack.” However, domestic national security law is often defined by executive branch lawyers and only rarely challenged in courts, and international law has limited recourse for a powerful nation like the United States.

“People want the law to do a lot of work here that perhaps strategy and policy need to do,” said Joshua Geltzer, a lawyer with the National Security Council during the Obama administration who is now a law professor at Georgetown. He said he doubted the strike could face serious legal challenge.

Hathaway argued, however, that Trump’s strike was probably unconstitutional. That the administration did not notify the “Gang of Eight” bipartisan U.S. senators, as well as Trump’s continued threats to Iran in the wake of the military operation, would diminish the argument that the strike on Soleimani was carried out in self-defense.

But she also said it was unlikely that courts would step in. “Congress really is the only bulwark against the president who wants to unilaterally use force,” she said.

“It doesn’t matter if it was lawful if it was a terrible idea,” Chesney said. “It’s a huge mistake to equate legality with wisdom.”