A week now after the initial revelation that the United States may have monitored the cellphone of German Chancellor Angela Merkel, there's little doubt that the story has been damaging for this country and for the National Security Agency, which earned the wrath of even longtime defender Sen. Dianne Feinstein, who oversees it as the Senate Intelligence Committee chair. At the same time, though, the initial anger appears to be giving way to debate: Is it, in fact, a bad idea for the United States to spy on friendly foreign leaders such as Merkel?

That question might sound counterintuitive, even cynical, a sign of the depth of Americans' hubris that we would even consider it. After all, friends don't spy on each other, right? But I'm going to let you in on a little secret: The international system is, and always has been, inherently adversarial, even among allies. To paraphrase the 19th-century British statesman Lord Palmerston, countries don't have friends, they have interests. Spying on friendly foreign nations does not actually violate the standard practices of international relations and in many ways is consistent with those norms. The close U.S. allies France and Israel are particularly known for it. Still, something as explicit as tapping Merkel's cellphone is a big and legitimately surprising step, one that may well go too far. Here is an evaluation of the pros and cons involved that may help clarify why the United States would decide to take such a step.

The simplest case for spying might be that the United States and Germany, despite being allies, still compete with one another, sometimes on quite substantive issues. If spying can give them a leg up on those issues, then aren't their leaders obligated to sanction it? President Barack Obama's job, after all, is to further American interests, Merkel's to further German interests. Those conflict more than you might think; when they do, both leaders are potentially better served if they spy on the other.