This is the way of the secret service: Between friends, we “exchange information” at least as much as we “monitor.” On Sunday, June 30, European Commission Vice President and Justice Commissioner* Viviane Reding affirmed a bit naively that, “between friends, we don’t spy on each other.”

But the information revealed by Der Spiegel — which no one has denied in Washington — is serious. According to the weekly German newspaper — whose source must undoubtedly be former American intelligence agent Edward Snowden, who was held at the Moscow airport and is in hiding in Russia — PRISM, the American spying program run by the National Security Agency, also targeted the European Union.

Its locations in Washington and New York were riddled with microphones and its computer networks infiltrated. Worse, the Europeans were described as “targets” in these documents, an incomprehensible description for allies. Also according to these documents, Germany is getting special attention, with 500 million phone and internet communications recorded each month.

Several European leaders, including the French, who are indignant, are demanding an explanation. American authorities let it be known that they would respond “appropriately” to the EU — by using diplomacy. That’s not enough.

Having been entangled in these unraveling revelations concerning the extent of its electronic surveillance programs for a month, the Obama administration must, once and for all, tell the truth not only to its citizens but also to its allies. Europeans are more attached to private data protection than Americans. They are thus even more sensitive to this offense — committed by the information services of a country that is supposed to protect them.

Ms. Reding has even more reason to complain about the fact that, under pressure from Silicon Valley giants, she had to accept a dilution of European legislation concerning the protection of private information. This question is one of the contentious points in the negotiations over a free-trade treaty that start next week between the EU and the U.S.

Since Sept. 11, the U.S. has agreed to a security policy expansion that now seems to be completely unmonitored and against its own values. Considered by some to be a traitor, Mr. Snowden actually did his country a huge favor by revealing the extent of this excessive policy.

Integrated into our lives, the digital world constantly exposes our lives to interference by government leaders and Internet giants. And apparently American agencies are taking full advantage. We’d like to make sure that our secret service doesn’t do the same here with us.

* Editor’s Note: Reding’s full title is European commissioner for justice, fundamental rights and citizenship.