Xi Jinping was unequivocal: "We cannot lose even one inch of the territory left behind by our ancestors,” China's president told visiting U.S. Secretary of Defense Jim Mattis Thursday.

Xi was referring to the vast swathes of the South China Sea that Beijing claims for its own, despite international tribunal rulings to the contrary.

Mattis had arrived in Beijing to deliver what a pentagon official told Financial Times would be a "medium-tough" warning over China's militarization of the South China Sea, but Xi clearly likes his steaks on the rarer, if not quite bloody, side.

In any case, senior U.S. officials have conceded that China's weaponization of various artificial islands is a fait accompli, so quite what the Pentagon chief thought Xi was going to say in response is anybody's guess.

Besides, Mattis sat down with XI with their two countries at loggerheads over trade, and needing the Communist Party chairman's support over sanctions and broader efforts to maintain the outwardly positive momentum over North Korea. You'll need to have a better hand than that to bluff Xi Jinping.

Yet despite being so heavily compromised, Mattis has being talking tough on the issue, warning that a White House move to disinvite China from a multinational naval exercise -- known as Rim of the Pacific -- this summer was "a "relatively small consequence" and adding " I believe there are much larger consequences in the future."

Whatever those consequences will be, they will have to amount to more than "medium-tough" table talk.

Editor: David Green