[A]s a matter of international law, the United States is in an armed conflict with al-Qaeda, as well as the Taliban and associated forces, in response to the horrific 9/11 attacks, and may use force consistent with its inherent right to self-defense under international law. As a matter of domestic law, Congress authorized the use of all necessary and appropriate force through the 2001 Authorization for Use of Military Force (AUMF). These domestic and international legal authorities continue to this day.
As recent events have shown, al-Qaeda has not abandoned its intent to attack the United States, and indeed continues to attack us. Thus, in this ongoing armed conflict, the United States has the authority under international law, and the responsibility to its citizens, to use force, including lethal force, to defend itself, including by targeting persons such as high-level al-Qaeda leaders who are planning attacks. As you know, this is a conflict with an organized terrorist enemy that does not have conventional forces, but that plans and executes its attacks against us and our allies while hiding among civilian populations. That behavior simultaneously makes the application of international law more difficult and more critical for the protection of innocent civilians. Of course, whether a particular individual will be targeted in a particular location will depend upon considerations specific to each case, including those related to the imminence of the threat, the sovereignty of the other states involved, and the willingness and ability of those states to suppress the threat the target poses. In particular, this Administration has carefully reviewed the rules governing targeting operations to ensure that these operations are conducted consistently with law of war principles, including:
• First, the principle of distinction, which requires that attacks be limited to military objectives and that civilians or civilian objects shall not be the object of the attack; and
• Second, the principle of proportionality, which prohibits attacks that may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, that would be excessive in relation to the concrete and direct military advantage anticipated.
In U.S. operations against al-Qaeda and its associated forces … great care is taken to adhere to these principles in both planning and execution, to ensure that only legitimate objectives are targeted and that collateral damage is kept to a minimum.…[S]ome have suggested that the very act of targeting a particular leader of an enemy force in an armed conflict must violate the laws of war. But individuals who are part of such an armed group are belligerents and, therefore, lawful targets under international law. During World War II, for example, American aviators tracked and shot down the airplane carrying the architect of the Japanese attack on Pearl Harbor, who was also the leader of enemy forces in the Battle of Midway. This was a lawful operation then, and would be if conducted today. Indeed, targeting particular individuals serves to narrow the focus when force is employed and to avoid broader harm to civilians and civilian objects.…
[In addition] some have argued that the use of lethal force against specific individuals fails to provide adequate process and thus constitutes unlawful extrajudicial killing. But a state that is engaged in an armed conflict or in legitimate self-defense is not required to provide targets with legal process before the state may use lethal force. Our procedures and practices for identifying lawful targets are extremely robust, and advanced technologies have helped to make our targeting even more precise. In my experience, the principles of distinction and proportionality that the United States applies are not just recited at meetings. They are implemented rigorously throughout the planning and execution of lethal operations to ensure that such operations are conducted in accordance with all applicable law.
… Finally, some have argued that our targeting practices violate domestic law, in particular, the long-standing domestic ban on assassinations. But under domestic law, the use of lawful weapons systems—consistent with the applicable laws of war—for precision targeting of specific high-level belligerent leaders when acting in self-defense or during an armed conflict is not unlawful, and hence does not constitute “assassination.”
Given bin Laden’s unquestioned leadership position within al Qaeda and his clear continuing operational role, there can be no question that he was the leader of an enemy force and a legitimate target in our armed conflict with al Qaeda. In addition, bin Laden continued to pose an imminent threat to the United States that engaged our right to use force, a threat that materials seized during the raid have only further documented. Under these circumstances, there is no question that he presented a lawful target for the use of lethal force. By enacting the AUMF, Congress expressly authorized the President to use military force “against … persons [such as bin Laden, whom the President] determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001 …in order to prevent any future acts of international terrorism against the United States by such … persons”. Moreover, the manner in which the U.S. operation was conducted—taking great pains both to distinguish between legitimate military objectives and civilians and to avoid excessive incidental injury to the latter—followed the principles of distinction and proportionality described above, and was designed specifically to preserve those principles, even if it meant putting U.S. forces in harm’s way. Finally, consistent with the laws of armed conflict and U.S. military doctrine, the U.S. forces were prepared to capture bin Laden if he had surrendered in a way that they could safely accept. The laws of armed conflict require acceptance of a genuine offer of surrender that is clearly communicated by the surrendering party and received by the opposing force, under circumstances where it is feasible for the opposing force to accept that offer of surrender. But where that is not the case, those laws authorize use of lethal force against an enemy belligerent, under the circumstances presented here.
In sum, the United States acted lawfully in carrying out its mission against Osama bin Laden.
Excerpts, Harold Hongju Koh, The Lawfulness of the U.S. Operation Against Osama bin Laden, opinio juris