The incident began when five members of the United Red Army (URA), following a bloody purge that left 14 members of the group plus one bystander dead, broke into a holiday lodge below Mount Asama, taking the wife of the lodge-keeper as a hostage. A standoff between police and the URA radicals took place, lasting ten days. The lodge was a natural fortress, solidly constructed of thick concrete on a steep hillside with only one entrance, which, along with their guns, enabled the hostage-takers to keep police at a distance.
On February 28, the police stormed the lodge. Two police officers were killed in the assault, but the hostage was rescued and the URA radicals were taken into custody. The incident contributed to a decline in popularity of leftist movements in Japan.
In the second week of February 1972 at the compound, URA's chairman Tsuneo Mori and vice-chairman Hiroko Nagata (sometimes referred to as Yoko Nagata) initiated a violent purge of the group's members. In the purge, Nagata and Mori directed the beating deaths of eight members and one non-member who happened to be present. Six other members were tied to trees outside where they froze to death in the frigid mountain winter air. On February 16, police arrested Mori, Nagata, and six other URA members at the compound or at a nearby village. Five others, armed with rifles and shotguns, managed to escape, fleeing on foot through the mountains towards Karuizawa in nearby Nagano prefecture. The five fugitives were Kunio Bando, 25, a graduate of Kyoto University, Masakuni Yoshino, 23, a senior at Yokohama National University, Hiroshi Sakaguchi, 25, a dropout of Tokyo Suisan University, Jiro Kato, 19, and his brother Saburo Kato, 16.
The lodge's structure made it a stronghold. The lodge, named after nearby Mount Asama, was a three-story wood and concrete edifice built into the side of the hill atop an exposed base of steel-reinforced concrete. The upper floor was slightly larger than the two below, giving the lodge a mushroom appearance. The building towered over the steep, snow-covered slopes below and the lodge's windows had heavy outer storm shutters. The building's maze-like floor plan and narrow staircases made it easy for the defenders to block off movement inside the building. The radicals would spend most of their time on the uppermost floor, which contained a kitchen, dining room, tatami-mat sleeping room, and a commanding view of the surrounding valley and hills. The radicals placed large pieces of furniture and futon bedding around the doors and windows and secured them in place with wire. When Muta's husband returned and saw the barricades he realized what had happened and quickly notified police. The police immediately set up roadblocks and surrounded the lodge to cut off any avenues of escape for the radicals inside.
The police decided to wait to see if the radicals would surrender on their own. After three days without a surrender offer from the hostage-takers, the police shut off the electricity to the lodge and set up loudspeakers from which the parents of several of the radicals implored them to surrender, to no avail. One of the participating parents' son had been killed in the purge incident, but both the police and the parent were unaware of this because the purge had not yet become known.
On February 25, the police began to prepare to assault the lodge. A wrecking ball crane with an armored driver's compartment was positioned near the building and police armed themselves with ladders, heavy mallets, and chainsaws. Muta's husband implored the radicals by loudspeaker to release his wife, but was ignored. On February 27, the police used a baseball pitching machine to bombard the building with rocks to keep the hostage-takers awake all night.
The police moved into position for the assault at 8 a.m. on February 28 and issued a final ultimatum an hour later, which went unheeded by the radicals. At 10 a.m. the wrecking-ball crane began to batter the lodge's walls. The police cautiously approached the building and began to break through the barricades. By noon, the police had occupied the two lower floors, isolating the radicals and Muta on the top floor.
The police experienced difficulty in broaching the radicals' defenses on the top floor and hours later had not made much headway. The police directed high-pressure water hoses at the top floor, gouging-out large holes in the building's walls and drenching the radicals and Muta with cold water. During this time, the radicals kept up continuous gunfire on the assaulting police and threw homemade bombs at them. Two policemen, Shigemitsu Takami, 42, and Hisataka Uchida, 47, were shot and killed and 15 other policemen were injured. A civilian observer who intruded into the area without police permission was also shot, reportedly by the radicals, and fatally wounded.
As darkness fell, the police breached the top floor's barricades and captured one of the Kato brothers. The remaining four radicals burrowed into a pile of futon bedding and refused to surrender. As the police approached them, Kunio Bando shot one of the policemen, Masahiro Endo, in the eye. Endo lost the eye but survived. Eventually, at 6:15 p.m., 280 hours after the incident began, the remaining four radicals were taken into custody and Muta was rescued. Muta was cold but uninjured and told police that her captors had not mistreated her, although they had tied her to a bed during most of the standoff. That same evening, despondent over his son's behavior, Kunio Bando's father hanged and killed himself in his home in Ōtsu, a city near Kyoto.
At 9:40 a.m. on February 28, public broadcaster NHK began live, continuous coverage of the siege that lasted until 8:20 p.m. that night. Ratings for NHK's non-stop coverage averaged 50.8% and peaked at 89.7% at 6:26 p.m. Vehicle traffic was noticeably lighter throughout the day in Tokyo.
On August 8, 1975 the Japanese government released Kunio Bando and flew him to asylum in Libya in response to demands from URA members who had stormed the American and Swedish embassies in Kuala Lumpur, Malaysia and taken 53 hostages. Bando later is believed to have assisted in the hijacking of Japan Airlines Flight 472 from Paris to Tokyo in 1977, forcing the jet to land in Dhaka. Bando remains at large and reportedly spent time between 1997 and 2007 in Russia, China, the Philippines, and Japan.
Yasuko Muta remained in the Karuizawa area, working at another lodge. She refused to speak any further about her ordeal since her initial statements to the police and press after she was rescued.
The incident, along with the Lod Airport massacre which occurred several months later, contributed to an intense social backlash among the population of Japan against radical student leftist groups. Since the incident, the leftist movement in Japan has greatly decreased in numbers and enjoys much less popular support.