The roots of the Asama-Sanso incident stretched back a decade. During the 1960s, leftist student movements swept through Japanese universities much as they did in the West, but by the late 1960s these groups had splintered into increasingly extreme factions. Two of the most radical -- the Keihin Anti-Security Treaty Joint Struggle Group and the Red Army Faction -- merged to form the United Red Army. The Keihin group had raided a gun shop in Mooka, Tochigi Prefecture, in February 1971, seizing nine shotguns, a rifle, and 2,300 rounds of ammunition. The Red Army Faction, led by Tsuneo Mori and including Kunio Bando, carried out a string of robberies -- four banks, three post offices, and an elementary school -- over five months. Hunted by a nationwide police dragnet, both groups fled to mountain hideouts in Gunma Prefecture, establishing bases on the slopes of Mount Haruna, Mount Kasho, and Mount Myogi.

What happened next was worse than anything the police would encounter at the lodge. In the second week of February 1972, URA chairman Tsuneo Mori and vice-chairman Hiroko Nagata imposed a brutal internal purge at the Gunma compound, demanding ideological self-criticism sessions from every member. Eight people were beaten to death on their orders. One was not even a member of the group -- just someone who happened to be present. Six more were tied to trees in the freezing mountain winter and left to die of exposure. On February 16, police raided the compound and arrested Mori, Nagata, and six others. But five armed fugitives escaped into the mountains: Kunio Bando, 25, a Kyoto University graduate; Masakuni Yoshino, 23; Hiroshi Sakaguchi, 25; and two brothers, Jiro Kato, 19, and Saburo Kato, just 16 years old.

On February 19, the five fugitives spotted police pursuit near Karuizawa and forced their way into Asama Sanso, a vacation lodge owned by Kawai Musical Instruments. Inside they found Yasuko Muta, the 31-year-old wife of the caretaker, alone -- her husband was walking the dog, the guests had gone ice skating. They took her hostage at gunpoint and barricaded the building. The lodge was a nightmare for any assault team: a three-story wood-and-concrete structure built into a steep hillside, with a reinforced concrete base, heavy outer storm shutters, a maze-like interior, and narrow staircases. The upper floor was slightly larger than the two below, giving the building a mushroom silhouette. It contained a kitchen, dining room, and tatami sleeping room -- and a commanding view of every approach through the valley. Police surrounded the lodge and cut off escape routes, but the building's single entrance and thick walls kept them at bay for days.

For three days, police waited for a voluntary surrender. When none came, they cut the electricity and brought in loudspeakers. Parents of the radicals were brought to the scene to plead with their children. One father was begging his son to come out, not knowing the son had already been killed during the purge. On February 27, a baseball pitching machine was used to bombard the building with rocks through the night. The assault began at 10 a.m. on February 28. A crane swung a wrecking ball into the lodge walls while tactical teams advanced with ladders, mallets, and chainsaws. By noon, police held the two lower floors. The top floor held out for hours longer, the radicals firing continuously and throwing homemade bombs. Inspector Shigemitsu Takami, 42, and Superintendent Hisataka Uchida, 47, were shot and killed. At 6:15 p.m. -- 280 hours after the standoff began -- police captured the last four radicals burrowed in a pile of futon bedding. Muta was cold but uninjured. NHK's live audience peaked at an 89.7% share at 6:26 p.m.

The consequences rippled outward. Four of the five were sentenced to long prison terms; Sakaguchi received a death sentence, which the Supreme Court of Japan upheld in 2013. Bando's story took the strangest turn: in 1975, Japanese Red Army members stormed the U.S. and Swedish embassies in Kuala Lumpur and took 53 hostages, demanding his release. The Japanese government complied, flying him to asylum in Libya. He is believed to have assisted in the hijacking of Japan Airlines Flight 472 in 1977 and remains at large. Muta, the hostage who survived unharmed, was vilified by gossip magazines that twisted her words to imply sympathy for her captors, and she never spoke publicly about the ordeal again. But the deepest impact was political. The televised spectacle of the siege -- combined with the revelation of the internal purge and the Lod Airport massacre carried out by Japanese Red Army members months later -- destroyed public sympathy for the radical left. The movement that had once filled university campuses collapsed. A 2007 film by Koji Wakamatsu, United Red Army, won the Japanese Eyes Best Picture award at the Tokyo International Film Festival, but the era it depicted was long dead. Media footage of police officers eating cup noodles during the standoff is credited with popularizing instant noodles as emergency food across Japan -- perhaps the siege's most unexpected legacy.