Donald Trump was apparently captivated by the “beautiful” roses in the garden of the Communist Party’s leadership compound when he visited Beijing in May. It was a relaxed moment between the US and Chinese leaders – otherwise locked in disputes ranging from trade to technology and Taiwan – that led to a new opportunity for gift diplomacy between the rival powers. Strolling around the former imperial garden at Zhongnanhai with Xi Jinping, Trump was so taken with a China rose variety in bloom that...