The administration has cited, as legal authority, the 2002 Authorization for Use of Military Force Against Iraq Resolution, which approved the invasion of Iraq. The resolution is still in place, granting legal powers as if the war had never ended.

If the administration can demonstrate that Mr. Suleimani’s activities in Iraq made him an adversary in that conflict, it can call on broad wartime authority to target him, Mr. Anderson said.

The administration has mostly emphasized claims that Mr. Suleimani posed an “imminent threat” to American lives, hinting at legal precedents set by past presidents.

The Bush and Obama administrations concluded that they could, under certain conditions, lawfully kill someone who posed an imminent threat — or whose past actions suggested they could pose a future threat. Their findings, which drew on interpretations of domestic and international law as permitting attacks to halt imminent threats, formed the basis of much of their targeted killing programs. However, there is evidence that the United States government uses an expansive definition of “imminent,” and many dispute whether it actually meets international legal standards.

Mr. Anderson said that killing Mr. Suleimani would almost certainly have met legal standards used by the Obama administration, calling him “targetable” for his role in overseeing past proxy attacks against American forces.

The Trump administration, however, initially said its strike was to deter future attacks, not to stop one. It has provided little evidence for its claims of an imminent threat, and some officials privately say that the case is thin and may not represent Mr. Trump’s actual motivation.