A few weeks ago, five Chinese vessels, two of them fishing trawlers, surrounded a US naval ship, the Impeccable, off Hainan island in the South China Sea. When the US survey ship responded with fire hoses, the Chinese crewmen stripped down to their underwear and – according to some reports – bared their bottoms.

The slightly surreal stand-off, which drew a sharp protest from Washington, was carefully calibrated. Though it fell well short of a military exchange, it nevertheless sent a message that Beijing was not prepared to tolerate routine US spying missions in waters it considers its own.