The issue of the large U.S. military presence in Okinawa is divisive, deeply rooted and, frankly, one I have never completely understood. Anti-base protests have been going on for decades, and while locals elsewhere in the developed world may have been unhappy with the bases in their vicinity, the Okinawans stand out for the tenacity and, at times, ferocity of their opposition. What keeps them going?

John Junkerman’s documentary “Okinawa: The Afterburn” (“Okinawa: Urizun no Ame”) sheds more light on this question than any of the other Okinawan-themed films I have seen, fiction or nonfiction.

As a former Okinawa resident who has lived in Japan for nearly four decades, Junkerman is unabashedly on the side of the protesters (in a program note he describes the Okinawan islands as “spoils of war”), but he presents both sides without strident editorializing.

He has also found archival footage and living witnesses to Okinawa’s troubled history, which illuminate — far more brightly than the standard journalistic regurgitation of facts and figures — why Okinawans continue to resist the bases 70 years after the Battle of Okinawa began on April 1, 1945.

The film begins with an account of that battle, accompanied by interviews with elderly but articulate survivors — Japanese, Okinawans and Americans.

One survivor of the battle is former Okinawa governor and anti-base activist Masahide Ota. “A lot people here say that battle still continues. That has certainly been true for me,” he says.

Another is Masa Inafuku, who served as a 17-year-old student nurse at the height of fighting. “There was no place in the world where the fighting was so futile,” she says.

Still another is Kamado Chibana, who was 26 when 83 civilians hiding in a cave with her committed group suicide rather than surrender to the Americans, despite assurances from Japanese-fluent soldiers that they wouldn’t be harmed.

These and other testimonies are illustrated by rarely seen color footage of the battle and its aftermath, showing soldiers with what former sergeant Leonard Lazarick describes as “thousand-yard stares,” as well as emaciated Okinawan civilians on the brink of collapse.

The film follows the story of these survivors and the succeeding generations, as Okinawa became a key launch pad for U.S. wars in Asia and the Middle East.

In the 1960s, local activists campaigned for the reversion of Okinawa to Japan from the U.S., who were then ruling the island as a military protectorate. “Japan didn’t fight wars, had no nuclear weapons and its economy was booming,” explains Eiko Asato, a writer who was a teenage activist at the time.

After Okinawa was returned to Japan in 1972, however, locals soon realized that the bases would remain, with the ruling Liberal Democratic Party’s support, together with all the problems that had long accompanied them. The film focuses on sexual violence perpetrated by U.S. soldiers on Okinawan women and others in the military, male and female. Witnesses from both sides testify, including a former soldier involved in a highly publicized 1995 group rape of a 12-year-old Okinawan girl.

The film concludes with an overview of ongoing disputes, including the protests over the construction of a new U.S. base in Nago’s Henoko district, which — after the intense opening sections — feels slightly scattershot. But it’s hard to neatly tie up the 70-year history of the U.S. military on the island with a bow, as the struggle continues.

Despite its English subtitle, “Afterburn” — a reference to Okinawa’s long postwar trauma — the film does not view the struggle as never-ending. The Japanese title, “Urizun no Ame,” means “the rains that herald spring” — the season of hope.