The Florida summit began with President Trump's grandchildren singing a Chinese song to President Xi and his wife, but even before the dinner plates were cleared, the host had created an awkward challenge for his guest.

China's position on Syria is much closer to Russia's than to the US. And the Chinese government will guess that the timing of the American missile strike was a blunt message that without more robust Chinese help on dismantling Kim Jong-un's nuclear programme, the next target for pre-emptive American military action might be North Korea.

In Chinese protocol, sudden moves disrupting set piece occasions are avoided wherever possible, and in private, President Xi is likely to be angry that President Trump chose to strike on the very night of his visit.

But in public Xi stuck to his script about the virtues of co-operation while Trump insisted they'd formed an outstanding relationship and made great progress.

As the two presidents went into their final lunch there were no specifics, and in the aftermath of the US missile strike on Syria, all the big questions which bedevil the US China relationship have be kicked down the road for President Trump's visit to China later this year.