Takeshi Onaga, who as governor of Okinawa led a passionate campaign against the U.S. military presence on the Japanese island, where an American base has been located since the close of World War II, died Aug. 8 at a hospital in Urasoe, Japan. He was 67.

The newspaper Ryukyu Shimpo, among other Japanese news outlets, confirmed his death. He had been operated on for pancreatic cancer in April and resumed work in May.

Mr. Onaga, who had lost a considerable amount of weight, said he was determined to fulfill his duties and live up to the expectations of Okinawans who supported his fight against a U.S. military base relocation plan and the heavy American troop presence on the small island.

Deputy Gov. Kiichiro Jahana said he was temporarily assuming Mr. Onaga’s duties. There will be an election within 50 days under Japanese local election rules.

Mr. Onaga was elected governor in November 2014 on a pledge to scrap plans to relocate a contentious U.S. Marine Corps air station, known as Futenma, to a less dense part of the island and close it instead. Opponents of the relocation plan say it only shifts the burden of hosting the facility elsewhere and the base should be moved off the island entirely.

A former conservative mayor of Naha, the capital of Okinawa Prefecture, Mr. Onaga often confronted top officials of the central government, saying Tokyo’s approach was highhanded and neglected the will of Okinawans. In 2015, four months after taking office, Mr. Onaga criticized Japanese Chief Cabinet Secretary Yoshihide Suga for “looking down on” Okinawans, citing Suga’s comment that the government planned to steadfastly go ahead with the relocation plan.

About half of the 50,000 American troops in Japan are stationed on Okinawa.

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Mr. Onaga filed a series of lawsuits against the central government, seeking a court injunction to stop a landfill at the planned relocation site. He was preparing another legal action when he died.

Mr. Onaga has said the Futenma problem dates back to the U.S. confiscation of Okinawan land after Japan’s World War II defeat. He said Tokyo’s postwar defense stance under the Japan-U.S. security alliance is built on Okinawa’s sacrifice.

“Okinawa has suffered a lot. Why do we have to suffer more?” he told The Washington Post in 2014. “What we are saying is that we want all of Japan to share the burden.” The prefecture, he added, makes up 0.6 percent of Japan’s land mass but is home to 74 percent of the U.S. military bases in the country.

The dispute over the relocation of the Futenma base also reflects centuries-old tensions between Okinawa and the Japanese mainland, which annexed the islands, formerly the independent kingdom of the Ryukyus, in 1878. In the final days of World War II, Okinawa became Japan’s only home battleground, and the island remained under U.S. rule for 20 years longer than the rest of Japan.

Mr. Onaga was born in Naha on Oct. 2, 1950, when Okinawa was still under U.S. occupation. A complete list of survivors was not immediately available.