COMMUTERS BETWEEN MARIN COUNTY and San Francisco in northern California are getting used to a new spectacle during rush hour. Vast, ungainly container ships, bearing China’s flag and name, plough along under the glorious Golden Gate Bridge. They are bringing goods into the Port of Oakland—and taking back America’s trade deficit. Any pleasure yachts zipping around the bay give them a wide berth.

This is China as a Pacific power, a commercial rather than a naval one. According to statistics gathered by Michael McDevitt, a retired rear-admiral at America’s Centre for Naval Analyses, it is now the world’s largest shipbuilder; has the third-largest merchant marine, and by far the largest number of vessels flying its own flag; and boasts a 695,000-strong fishing fleet. It accounts for about a quarter of the world’s container trade. And almost all the steel boxes shipped on the world’s oceans are made in China, too.

Much of the security of that trade across the Pacific is the gift of America. China “free-rides” on the protection provided by the United States Pacific Fleet, based in Pearl Harbour, Hawaii, so it benefits from America’s enforcement of the rules of sea-based activity. But in the western Pacific China has behaved provocatively towards some staunch American allies, testing the bounds of international maritime law.

…Euan Graham of the Singapore-based S. Rajaratnam School of International Studies says that eventually these could enable it to thicken its EEZ into a robust coastal buffer. He notes that Chinese history—such as Britain’s shameful Opium wars of the 1840s and 1850s—makes the country particularly sensitive to maritime threats.