Nitta Yoshisada's decision to defy the Shogunate was not an act of sudden courage but a calculated bet. The Genko War had been grinding on since 1331, with Emperor Go-Daigo's supporters fighting to restore direct imperial rule and break the power of the Hojo clan, who had controlled the Shogunate as regents for generations. Yoshisada received an imperial mandate and saw his chance. He gathered warriors from his home province of Kozuke and began marching south toward the Shogunate's capital at Kamakura, collecting allies along the way. By the time his army reached Kotesashi moor, it had grown large enough to alarm the Shogunate commanders sent to stop it. The two forces drew up on opposite sides of the Iruma River on the morning of May 11, each watching the other across the water.

The battle opened in traditional samurai fashion: an archery barrage. Yoshisada's forces sent forward a hundred bowmen, and the Shogunate replied with two hundred archers of their own, followed by an assault of a thousand mounted warriors. Yoshisada countered with two thousand cavalry, and the engagement escalated into a full-scale battle. According to the Taiheiki, the great medieval chronicle that recorded these events, the two sides clashed more than thirty times over the course of the day, waves of mounted samurai charging and withdrawing across the moor. The fighting was fierce but ultimately inconclusive. By nightfall, both sides withdrew to count their losses. The imperial forces, having lost roughly three hundred dead, camped beside the Iruma River. The Shogunate army, with about five hundred killed, pulled back five kilometers south to the Kume River.

The standoff lasted one night. On May 12, the armies clashed again at the Battle of Kumegawa, and this time the imperial forces gained a clear advantage, pushing the Shogunate defenders further south. What had begun as a single rebel's march from Kozuke had become an unstoppable advance. In less than a week after Kotesashi, Yoshisada's army covered the remaining fifty kilometers to Kamakura itself. The Siege of Kamakura that followed was catastrophic for the Hojo regents. The city fell, the last Hojo leaders committed suicide, and the Kamakura Shogunate -- which had governed Japan since 1185 -- was destroyed. The battles at Kotesashi and Kumegawa were the hinge: the first moments when the Shogunate's military machine met a force it could not turn back.

Today, the site of the Battle of Kotesashi lies within the modern city of Tokorozawa, a commuter suburb of Tokyo known more for its aviation museum and baseball stadium than for medieval warfare. The Iruma River still flows through the area, but the moor where samurai cavalry charged in waves is long buried under residential streets and shopping districts. The landscape gives no hint of the violence that unfolded here seven centuries ago. Yet the battle's consequences shaped the entire arc of Japanese history. The fall of the Kamakura Shogunate led to Emperor Go-Daigo's brief restoration of imperial power, which in turn collapsed into civil war and the rise of the Ashikaga Shogunate. Kotesashi was the first domino. Standing on an ordinary sidewalk in Tokorozawa, you are standing where Japan's political order cracked open.