– On a cloudless afternoon, thousands of parents, teachers and alumni watched as a pack of young students charged across a field, screaming and snarling, and then crashed into a wall of students defending an 12-foot wooden pole. The attackers clawed, shoved and jumped over the opposition. Heads butted. Elbows were thrown. The wall buckled, then stood firm. Like a mast on a sailboat in rough seas, the pole dipped, then rose again.

This wasn't trench warfare, it was botaoshi, a century-old game that combines elements of American football, rugby, sumo and martial arts. The game has gotten so dangerous that many Japanese schools have abandoned it, but it lives on at Kaisei Gakuen, where it is the centerpiece of the school's annual sports festival.

Botaoshi, or "topple the pole," remains a rite of passage at Kaisei, which opened in 1871 and is one of Japan's most prestigious secondary schools. Teachers say the game promotes teamwork, toughness and sportsmanship. Students eagerly await their chance to compete in the tournament in their junior and senior years. Alumni can recount details of games played decades ago.

"Since my first year in junior high school, I was watching my senpai," Makoto Nakagawa, a recent Kaisei graduate, said of his upperclassmen. "The image of botaoshi was at the center of everything. It's a tradition."

To the uninitiated, botaoshi looks like mass chaos. At one end of a field, two dozen attackers try to pull down a pole while two dozen defenders try to keep it upright. At the other end of the field, the mirror image unfolds, except the attackers are on the same team as the defenders 50 or so feet away, and the defenders are teammates with the attackers.

The first team to lower the tip of the pole to below 140 centimeters (roughly 4½ feet) from the ground wins. If neither team succeeds within 90 seconds, they start over.

The players wear little protective gear besides soft helmets and kneepads. Sprains, cuts and bloody noses are common. Every so often, students may fracture leg bones, vertebrae and cheekbones or suffer a concussion. After practice, students pile into the nurse's office for ice packs and bandages.

The number of injuries in botaoshi has risen in recent years. According to school records, the number of injuries jumped 52 percent from 2005 to 2016, the most recent year for which records were available.

The increase can partly be attributed to better reporting, said Tetsuo Shimizu, a longtime math teacher at the school and a spokesman for the festival, but also to the fact that students who don't participate in any other athletics are increasingly joining in botaoshi.

"Students who are in sports clubs and students in art and literature clubs who never do sports are all mixed together," said Satoshi Matsumoto, a Kaisei graduate and doctor who volunteered at the sports festival. "It is impressive that there are no serious accidents."

Minoru Matsunami, a sports historian at Tokai University, said botaoshi might be a combination of several recreational games popular in the 1890s, such as sao nobori (pole climbing), hata tori (capture the flag) and tsuna nobori hata tori (climbing a rope and grabbing a piece of cloth).

Conditions were rougher years ago. At Kaisei, games would typically last about five minutes, there was virtually no protective gear and only a handful of rules.

"It was more savage and loose," said Akito Hattori, who graduated from Kaisei in 1977 and broke his clavicle as a defender playing botaoshi. Shin Sone, his best friend from that era who was an attacker in botaoshi, was blunter. "It was just a head-on fight," he said.

By the 1980s, parents pushed their children to focus more on college entrance exams and became concerned that botaoshi was too dangerous. Schools began dropping the sport. A fear of lawsuits accelerated the trend.

Why, then, does the game endure at Kaisei?

Matsunami said it remains a good way to teach teamwork and build stamina, even if it looks chaotic. To someone from Japan watching American football, "a lot of people would think it's crazy," he said. "It's the same thing."