Experts and politicians are debating if the U.S. killing of Iranian Gen. Qassem Soleimani and a top Iraqi militia commander on Friday was legally justified. But the strike is forcing a bigger question: If Donald Trump can get away with this, what can’t the president do?

The closest thing to a legal justification offered by the U.S. government so far is a Pentagon statement asserting that “This strike was aimed at deterring future Iranian attack plans.” According to Joint Chiefs Chairman Gen. Mark Milley, the U.S. had “clear and unambiguous evidence“ that Soleimani was planning a stepped up “campaign of violence.”

The vagueness of that threat falls short of the traditional understanding of self-defense.

“Usually when you’re talking about limited self-defense actions, you’re talking about a pretty concrete, finite threat. You’re only supposed to take a proportionate response to get yourself out of harm’s way,” says Scott Anderson, a former legal adviser at the State Department who is now a fellow at the Brookings Institution. Anderson notes that the U.S. has often taken a somewhat more expansive view of self-defense. “This goes one step even further saying we’re taking preemptive action against people who are planning threats against us. It’s just on a different scale.”

It’s hard to judge the extent of any threat without access to the U.S. government’s intelligence. But Soleimani was involved in attacks against U.S. troops in the region for decades, and the U.S. has so far avoided taking this step. So why did killing him now become an absolute military necessity? Even Milley acknowledged that an attack “might still happen” without Soleimani.

The Trump administration has undoubtedly stretched the president’s war-making authority, but it had already gotten pretty stretchy over the past two decades. Like the two presidents before him, Trump will almost certainly trot out a pair of well-worn resolutions to justify his actions. The 2001 Authorization for the Use of Military Force authorized action against the perpetrators of the 9/11 attacks, but has been applied to a growing number of countries and groups since then—and the 2002 AUMF authorized military action in Iraq.

According to a letter from the State Department to Congress last year, the Trump administration interprets the two AUMFs as permitting military action against Iran if it is “necessary to defend U.S. or partner forces engaged in counterterrorism operations or operations to establish a stable, democratic Iraq.” A State Department official said earlier this week that the strikes the U.S. carried out last Sunday against the Iranian-backed militias Kataib Hezbollah were justified under this interpretation of the AUMF.

But the killing of Soleimani, one of the most powerful people in Iran, is on another level of magnitude, raising the question of just how far this authority goes. Could the U.S. bomb Tehran in order to defend its counterterrorism mission? Launch a ground invasion?

All three presidents since 9/11 have made innovative use of the 2001 AUMF to justify military action with only the most tenuous connection to 9/11 or al-Qaida. Barack Obama made some halfhearted attempts to repeal the law, but also used it to attack ISIS—a group that didn’t exist on Sept. 11, 2001, and opposes al-Qaida.

The AUMF is not the only tool expanding executive war powers. To justify missile strikes against Syria in 2018, the Trump administration cited Obama’s Office of Legal Counsel’s argument for the legality of the 2011 intervention in Libya: that “constrained and limited operations” did not constitute hostilities under the War Powers Resolution, and therefore do not require congressional authorization. As I wrote when Trump was considering military action against Iran last summer, it’s easy to imagine such “limited” strikes being justified the same way. (In the Soleimani case, Trump didn’t even inform congressional leaders, much less seek authorization, though Sen. Lindsey Graham says he was apparently filled in.)

Congress deserves fault for failing to exercise oversight and letting war powers expand to this degree, and the situation seems unlikely to improve any time soon. Sen. Tim Kaine, a longtime critic of executive power, says he will introduce a new war powers resolution to force a debate on hostilities with Iran, but it’s unlikely to pass in a Republican-controlled Senate. Meaningful war powers reform will require members of Congress who are willing to limit the powers of their own party’s president.

Beyond the question of what U.S. law allows, international law is also getting murkier. The U.S. troops in Iraq, including, presumably, those that launched the strike on Soleimani, are there at the invitation of the Iraqi government for the specific purpose of fighting ISIS—not Iran. Launching a military operation on another country’s soil without that country’s government’s permission is generally considered an act of aggression. The U.S. has justified such actions in the past (think of the operation in Pakistan to kill Osama Bin Laden, or the fight against ISIS in Syria) when the government in question has proved “unwilling or unable” to deal with a threat such as terrorism.

That’s relatively new and dicey territory. (As recently as 2008, John McCain—not exactly a dove—criticized Barack Obama as irresponsible for saying he would strike al-Qaida targets in Pakistan without the government’s permission. “That’s still bombing Pakistan,” he said.)

Complicating things further, one of the men killed, Abu Mahdi al-Muhandis, was both a militia leader and an Iraqi government official. Iraq’s prime minister has said he views the killing as an act of aggression against the Iraqi state. Iraq could make a case that it now has a legal right to respond militarily. At the very least, Trump’s actions have put the future status of U.S. forces in Iraq in question.

As for Soleimani, he may have the blood of hundreds of American service members on his hands, and opposing commanders are considered fair game in war. But killing a country’s senior general is the sort of thing you only do when you are in a state of war or outright hostilities, which is why commentators including Slate’s Fred Kaplan have concluded that we’re now at war.

The Trump administration is unlikely to agree with this framing. “If you accept that we’re in a state of armed conflict with Iran then actions against us aren’t necessarily terrorism, maybe they’re acts of war,” says Anderson. “Are they then allowed to legitimately target our military leaders?”

In Trump’s defense, Iran is aware of these standards and has consistently exploited them. Figures like Muhandis are state officials working with nonstate actors, with often blurry and shifting loyalties, making them hard to fit into a traditional legal framework. Iran rarely attacks its enemies directly. It acts through proxies—Iraqi and Syrian militias, Hezbollah, Yemen’s Houthis—and maintains just enough plausible deniability to avoid provoking direct retaliation. Developing this type of action defined Soleimani’s career. In the recent actions by Iranian proxies in Iraq, he miscalculated by overestimating Trump’s desire to avoid war. But if nothing short of a claimed attack by official Iranian personnel warrants a military response, then Iran will continue to get away with an awful lot.

Still, that doesn’t justify the dangerous escalation we’ve just seen, and the fact that old standards seem outdated doesn’t mean they should be thrown out entirely. Countries rarely formally declare war on each other anymore. While the U.S. has turned the entire world into a borderless counterterrorism battlefield, adversaries like Iran and Russia have become adept at fighting back by working in gray areas through proxies and unacknowledged covert actions. This requires a thoughtful reconsideration of the rules of war, not simply surrendering to anarchy.