BEIJING, June 16 -- The trade war over the world's most advanced chips has quietly moved off the docks and into the courtroom, and over the weekend China's highest court delivered its verdict. The Supreme People's Court barred Infineon, the German firm that ranks among the largest power-chip makers in the world, from selling a class of its gallium nitride products anywhere in mainland China, the single biggest market on earth for the components that feed everything from phone chargers to the power racks behind artificial intelligence.

The ruling handed a decisive home victory to Innoscience, a Suzhou company that has grown into the world's largest maker of gallium nitride chips, and it crystallized something the export-control fight had ...