Former Japanese prime minister Yasuhiro Nakasone, a giant of his country’s post-World War II politics who pushed for a more assertive Japan while strengthening military ties with the United States, died Nov. 29 at a Tokyo hospital. He was 101.

The office of his son, Hirofumi Nakasone, a member of the Japanese parliament, confirmed the death. The cause was not disclosed.

As a World War II navy officer, Yasuhiro Nakasone witnessed the depths of his country’s utter defeat and devastation. Four decades later, he presided over Japan in the 1980s at the pinnacle of its economic success.

Mr. Nakasone began his political career as a fiery nationalist denouncing the U.S. occupation that lasted from 1945 to 1952, but by the 1980s he was a stalwart ally of the United States known for his warm relations with President Ronald Reagan.

He boosted defense spending, tried to revise Japan’s U.S.-drafted pacifist constitution and drew criticism for his unabashed appeals to patriotism.

In the 1950s, he was a driving force behind building nuclear reactors in resource-poor Japan, a move that helped propel the country’s strong economic growth after World War II but drew renewed scrutiny in the aftermath of the meltdowns at a nuclear plant in Fukushima swamped by a tsunami in 2011.

Mr. Nakasone was born May 27, 1918. His father was a lumber merchant.

After attending Tokyo Imperial University, Mr. Nakasone worked for the Japanese Interior Ministry and then entered the navy, where he rose to the rank of lieutenant commander during World War II.

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In his last news conference as prime minister, he said his political ambitions were sparked after the war by “the conviction I felt as I gazed bewildered at the burned ruins of Tokyo.”

“How can this country be revived into a happy and flourishing state?” he said.

He established his nationalist credentials by campaigning for parliament while riding a white bicycle bearing the rising sun of the Japanese national flag, which the country’s wartime military had used. He won a seat in 1947, becoming the youngest member of parliament at 28.

Mr. Nakasone became a leading figure in the Liberal Democratic Party that has dominated Japan’s postwar politics. During more than a half-century in parliament, he served as defense chief and headed the powerful Ministry of International Trade and Industry before becoming prime minister.

Mr. Nakasone assailed the U.S.-drafted postwar constitution, demanding revision of the document’s war-renouncing Article 9 and urging a military buildup.

He was a key figure behind providing government funding for nuclear research in 1954, less than a decade after the United States dropped atomic bombs on Hiroshima and Nagasaki, killing as many as 200,000 people in the last days of the war. In 1955, he helped pass legislation designed to promote nuclear power.

In a 2006 speech marking the 50th anniversary of Japan’s first nuclear institute, Mr. Nakasone said he was intrigued by nuclear power as he tried to figure out why Japan lost the war.

“My conclusion was that one of the biggest reasons was [the lack of] science and technology,” he said. “I felt strongly that Japan would end up being a lowly farming nation forever unless we take a bold step to develop science and technology.”

After the Fukushima disaster in 2011, there was a public backlash against nuclear energy, but Mr. Nakasone said it remained indispensable to maintain Japan’s industrial growth.

As prime minister from 1982 to 1987, Mr. Nakasone broke the mold of the Japanese politician, with an outspokenness that appealed to voters.

His tongue sometimes got him in trouble, and he sparked outrage in 1986 by suggesting Japan was an economic success because it did not have minorities with lower intellectual levels.

He was the first Japanese prime minister to visit South Korea, a country with bitter memories of its colonization by Japan from 1910 to 1945.

Mr. Nakasone was also staunchly committed to Japan’s alliance with the United States, exhibited by his friendship with Reagan.

His premiership coincided with a period of major trade disputes with the West. Responding to U.S. complaints, Mr. Nakasone initiated efforts to reduce tariffs and other import barriers.

He also privatized the sprawling Japanese railroad system, as well as the state telephone and tobacco companies.

In 1985, Mr. Nakasone was the first postwar prime minister to visit Yasukuni Shrine, which honors the war dead, including Japan’s convicted war criminals. His visit fueled disputes with China and South Korea over World War II history that persist to this day.

Mr. Nakasone overcame opposition from Japan’s strong pacifist forces to boost defense budgets and excluded military technology cooperation with the United States from Japan’s ban on arms exports.

He spoke of how Japan “cowered under the [postwar U.S.] occupation and occupation policies” but said the country should endeavor to stay away from warfare.

“We must stick to our commitment as a pacifist nation,” he said in a 2011 interview with public broadcaster NHK. “We have caused tremendous trouble to our neighboring countries in the past war. Our commitment to peace must be the centerpiece of Japan’s domestic and diplomatic policies.”

In later life, Mr. Nakasone became one of Japan’s leading elder statesmen. He promoted his longtime dream of revising the U.S.-drafted constitution and pronouncing his views on national and international affairs. He had attended an annual May rally campaigning for a constitutional revision until he skipped one just before turning 100, when he had a hand injury and could not use his cane to rise from his wheelchair.

His wife of 67 years, the former Tsutako Kobayashi, died in 2012. Survivors include three children and three grandchildren.

Mr. Nakasone retired from parliament in 2003, at age 85, after then-Prime Minister Junichiro Koizumi had urged him to step aside in upcoming elections to make way for a younger lawmaker.

Mr. Nakasone complied but accused Koizumi of discrimination and lack of respect for his elders. He publicly read a haiku poem. Years later, in his 100th-birthday message, Mr. Nakasone said the same haiku still best described his spirit:

Even after dusk,

Cicada persists in song,

While it still has life.

— Associated Press