Only 10 days ago, US defence officials described their dialogue with China as a love- fest. The two sides had just had their “best ever” talks, said David Sedney, the Pentagon's top official for China.

This week, a Chinese military leader called the US “villains” in response to accusations from the Pentagon that Chinese ships had harassed Impeccable, a US Navy survey vessel, in the South China Sea last weekend. Dennis Blair, the national intelligence director and former head of US Pacific Command, said the incident was the “most serious” since the collision of a US spy plane and a Chinese fighter jet over Hainan island in 2001.

After initial bursts of outrage, both sides seem determined not to allow the incident to damage broader relations. But it serves as a chilling reminder of the reality in military ties between the world's only superpower and the nation most likely to become its main challenger.