WE DON’T know what the National Security Agency (NSA) learned from tapping the cellphone of German Chancellor Angela Merkel. If the White House account is to be believed, no intercepts were deemed worth reporting to President Obama. But it is painfully clear that the damage from the revelation of the tap is considerable.

A high-level German delegation was set to arrive in Washington Wednesday to address what Ms. Merkel’s government has called a breakdown in trust; another delegation from the European parliament was already in town to make the same point. There is talk in Brussels and Strasbourg of cutting off NSA access to vital electronic networks , such as the SWIFT clearinghouse for bank transactions, and of imposing onerous requirements on private U.S. technology firms that handle European Internet traffic.

Mr. Obama seems already to have judged that the high-level bugging operation was a liability. He reportedly ordered it scaled back in August, and assured Ms. Merkel in a phone call last week that her communications were not being monitored now or in the future. That still seems to leave questions about the NSA’s spying on allied political leaders, which means there’s more cleaning up to do. How many presidents and prime ministers from NATO countries or major non-NATO allies have been targeted by the agency, and for how long? Who approved the taps, if not Mr. Obama? Was there ever a careful political review of whether such monitoring was worth the potential cost of disclosure?

It’s important to make a distinction between the NSA’s collection of bulk foreign Internet and phone data for counterterrorism investigations and the surveillance of political leaders. The former has helped to protect both the United States and its allies, including Germany, from al-Qaeda attacks; when it came to light last summer, Ms. Merkel helped to stifle controversy about it. The latter can be connected to counterterrorism or non-proliferation operations only with a great stretch. While there may be some circumstances where spying on a nominally friendly allied leader may be justified — former German chancellor Gerhard Schroeder’s collaboration with Russia’s Vladi­mir Putin to counter the George W. Bush administration comes to mind — it should be rare. And it should not happen without presidential authorization and disclosure to Congress.

Sen. Dianne Feinstein (D-Calif.), chairman of the Senate intelligence committee, made those points on Monday while disclosing that her committee had not been told about the political spying. Ms. Feinstein said her committee would “initiate a major review into all intelligence collection programs.” That shouldn’t lead to a witch hunt at the NSA, the dismantlement of vital collection operations in European countries or a surrender to the demands of European politicians, some of whose intelligence services are known to target the United States. But it should lead to the establishment of greater political control and accountability for sensitive foreign operations, starting with the president.