DARWIN, Australia—U.S. Marines use the commercial port in this northern Australian city to unload aircraft and other supplies for nearby training. But at the port gate, vehicles pass underneath a sign in Chinese. The sign, which says Landbridge Darwin Port in Chinese characters, is a reminder of a long-simmering controversy: the 2015 decision by local authorities to lease the port to the Australian unit of China’s Landbridge Group. Now, with competition between the U.S. and China intensifying in the Indo-Pacific, the 99-year lease—which some former defense officials and analysts say gives China a potentially valuable foothold in a strategically important area—is under renewed scrutiny.

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