The US National Security Agency has been repeatedly embarrassed this year by details of its extensive surveillance operations exposed by Edward Snowden, the former contractor. None of these revelations is diplomatically as damaging as his disclosure that the NSA has listened for most of the past decade to the mobile phone conversations of Angela Merkel, the German chancellor. The revelation has triggered deep anger in Berlin while the US has in effect admitted that the allegation is true.

Exactly why the NSA decided to start bugging Ms Merkel’s phone, and what intelligence it hoped to glean, remains a mystery. But the NSA’s decision, certainly after she attained power, was reckless. All nations spy on each other, and even on their allies. But in this case, any intelligence gains were bound to be minuscule when set against the immense damage that any future disclosure would inevitably do to US-German relations.

The damage is hard to overstate. This is not just the US bugging a head of government who happens to be a close ally and Europe’s pivotal leader. She also grew up under the German Democratic Republic and the surveillance of the Stasi secret police. She is right to regard this revelation as a serious breach of trust.