I was meant to feel that I was in big trouble. Nato takes its secrets seriously and I had just published a big one. A senior official explained grimly that a leak inquiry was under way. I and my sources should expect consequences.

My conscience and nerves were unbothered. The secret was that the alliance had just decided, in January 2010, to make plans for the defence of its most vulnerable members: Estonia, Latvia and Lithuania. The decision made perfect sense. Plans (also secret) already existed to defend Poland. Aggressive Russian military exercises had recently rehearsed the invasion of the Baltic states, concluding with a dummy nuclear strike on Warsaw.

The secrecy was political, not military. The Obama administration and the German government did not…