The question of the hour is: "How does international law limit the war on terror?" Constraints of space restrict this piece to the role that customary international law plays in the war. The answer to the more specific question--"How does customary international law limit the war on terror?"--is more complicated than one might suppose.
Customary international law (1) might limit America's conduct of the war in one of two ways. First, American politicians might choose to adhere to customary international law if they believe that such adherence is in America's best interest. Second, American politicians might believe that they are legally bound to respect international law, including customary international law, in the conduct of the war.
The notion that politicians would choose to respect customary international law in fighting the war seems dubious. One suspects that the principal players--the President, members of Congress, and cabinet secretaries--do not really even know what customary international law is. They are all familiar with the Constitution, statutes, and treaties of the United States because these are forms of law they encounter every day. But customary international law is a more obscure beast. Obviously, if politicians are generally unaware of customary international law, it cannot greatly limit their decision making.
To the extent that principals in the political branches are aware of customary international law, that awareness likely stems from the efforts of staff members who are committed to customary international law. Yet, politicians are likely to get conflicting signals from their staff regarding the status of customary international law. Those staff members who are skeptical of customary international law are likely to tell their principals that customary international law forms no part of United States law, and that even if it does, many claimed principles of customary international law are of doubtful vintage and provenance. Given the diversity of viewpoints about the status and content of customary international law, politicians are likely to view customary international law as something of a non-issue. Conflicting advice gives politicians the freedom to do whatever they wish.
Likewise, the overriding desire to defeat the enemy makes it even less likely that politicians will pay much heed to customary international law in the war context. The shadowy and uncertain principles of customary international law pale in comparison to the need for victory. For all these reasons, it is doubtful that customary international law limits the war on terror in any meaningful way. In the end, most politicians will not resist the urge to shove customary international law out of the way.
Finally, the Supreme Court has made it clear that both the President and Congress can break free of customary international law by simple decree. In The Paquete Habana, (2) after claiming that "[i]international law is part of our law," (3) the Court went on to declare that "where there is no treaty, and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations." (4) Paquete Habana thus provides a roadmap to abandon the constraints of customary international law: override it by passing contrary laws or overcome it by taking a contrary "controlling executive" action.
Of course, this speculation about voluntary adherence to customary international law leaves open the question whether politicians are legally obligated to respect customary international law. International law scholars have argued that American officials must respect customary international law because the Constitution makes that law part of the "supreme Law of the Land." (5) Those who claim that customary international law is part of the "supreme Law of the Land" always cite the middle phrase of the Supremacy Clause--"Laws of the United States which shall be made in Pursuance thereof. …