The official added that the administration also explored whether there was another way to stop Soleimani, such as having him arrested, and determined there was "no way."
Was killing Soleimani legal under U.S. domestic law? "It's clearly lawful," says John Bellinger, a fellow at the Council on Foreign Relations and former legal adviser to the Department of State under the George W. Bush administration. "It's clearly an exercise of the president's constitutional authority as commander in chief and chief executive to use force in the national interest."
The answer is less straightforward for Scott Anderson, a fellow at the Brookings Institution who served as the legal adviser for the U.S. Embassy in Baghdad during the Obama administration. He notes that in the U.S., "the legal views as to what is permitted by the law in terms of the use of military force really is pretty well-defined by the executive branch itself."
That's because Congress has not imposed substantive limits on the executive's use of force and federal courts have appeared reluctant to intervene, he says.
In recent years, some members of Congress have occasionally criticized the fact that complicated wars grinding on in the Middle East have been justified by legislation passed nearly two decades ago.
U.S. officials did not state a specific legal basis for the strike until Friday evening, when Trump's national security adviser, Robert O'Brien, cited the 2002 Authorization for Use of Military Force, which ushered in the war against Saddam Hussein's regime in Iraq. O'Brien also said that the authorization was consistent with the president's constitutional authorities as commander in chief to defend the nation and forces against attacks.
O'Brien did not elaborate on the nature of the threat. The strike against Soleimani was carried out without officially notifying Congress ahead of time.
Anderson says that the Trump administration's legal justifications are "adopting a stance that pushes a little beyond what it and prior presidential administrations ... have done in the past."
He says it's hard to argue that the action against Soleimani is completely unlawful under U.S. law – but said that justifications may rely on interpretations of law that are controversial or antiquated.
Jack Goldsmith, a professor at Harvard Law School, notes that presidential war powers have been steadily expanding for years – and that Congress has largely gone along with it. "Our country has, quite self-consciously, given one person, the President, an enormous sprawling military and enormous discretion to use it in ways that can easily lead to a massive war," he said on Twitter. "That is our system: one person decides."