One morning in February 1784, a converted privateer called the Empress of China sailed out of New York harbour. It was bound for a country almost no one aboard had ever seen – China, then ruled by the Qing dynasty. Six months later, it dropped anchor at Canton, the only port where the Qing government allowed Western maritime trade. At the time, the Declaration of Independence was less than a decade old, and the Treaty of Paris recognising American independence had been signed less than a year...