Operation U-Go aimed to capture Imphal, the capital of Manipur and a major Allied logistics base with airfields, supply dumps, and encampments. Three Japanese divisions -- the 33rd, 15th, and 31st -- would attack from the south, east, and north, cutting the road that wound 100 miles through the Naga Hills to the even larger Allied base at Dimapur. Success would sever the supply lines to the American-led forces building the Ledo Road and to the airlift sustaining Nationalist China over the Himalayas. The plan assumed victory within three weeks, before monsoon rains made the supply routes from the Chindwin River impassable. It assumed the British and Indian troops were the demoralized, poorly led forces Mutaguchi had routed in Malaya and Singapore two years earlier. Both assumptions proved fatally wrong. The Allies, under General William Slim, had rebuilt their armies into well-trained, well-supplied formations with near-total air superiority.

When Allied intelligence detected the coming offensive, Slim and his corps commander, Lieutenant General Geoffry Scoones, made a counterintuitive decision: withdraw their forward divisions into the Imphal plain and force the Japanese to fight at the end of impossibly long supply lines. The execution was messy. They misjudged the attack date, and the 17th Indian Division at Tiddim, 243 kilometers south of Imphal, was nearly cut off by the Japanese 33rd Division. The Indian troops fought their way back through ambushes and roadblocks, crossing the Manipur River and demolishing the bridge behind them. They reached the Imphal plain on 4 April, battered but intact. Meanwhile, Admiral Lord Mountbatten commandeered transport aircraft from the China airlift to fly the battle-hardened 5th Indian Division from the Arakan front. The entire division -- artillery, jeeps, mules, and all -- was airlifted to Imphal in eleven days, a logistical feat that transformed the defense.

The Japanese had assumed tanks could not climb the steep, jungle-covered hills around Imphal. They left behind most of their field artillery -- their primary anti-tank weapon -- for ease of movement. This proved catastrophic. When a battalion of the Japanese 51st Regiment seized Nungshigum Ridge overlooking Imphal's main airstrip, the 5th Indian Division counterattacked on 13 April with air strikes, massed artillery, and the M3 Lee tanks of B Squadron, 3rd Carabiniers. The Lee tanks had never been driven up such gradients in combat. They climbed anyway. The Japanese, nearly defenseless against armor, were driven from the ridge with heavy casualties. The cost was severe on both sides: every officer of the Carabiniers and the attacking 1st Battalion, 17th Dogra Regiment, was killed or wounded. At Shenam Saddle on the Tamu road, twelve Japanese tanks were caught exposed on the road by British anti-tank guns on 4 April. From that point, the Japanese never achieved the armored breakthrough they needed to reach Imphal's airfields.

By May, the offensive had stalled on every front. The monsoon broke, turning trails to mud. Japanese troops began abandoning their positions to forage in local villages. At a meeting on 6 June, Mutaguchi and his superior, Lieutenant General Kawabe, both used haragei -- the Japanese practice of unspoken communication through gesture and expression -- to convey their mutual conviction that success was impossible. Neither man would accept responsibility for ordering retreat. Kawabe fell ill with dysentery. Mutaguchi dismissed the mortally ill commander of the 15th Division and replaced him, but the new appointment changed nothing. The 33rd Division's commander was also relieved after his cautious approach infuriated Mutaguchi. Fresh troops sent to reinforce the southern attack suffered devastating casualties from artillery fire. When Mutaguchi finally ordered the offensive broken off on 3 July, his armies had disintegrated. Soldiers too sick or wounded to walk were abandoned along the retreat routes.

The Allies recaptured Tamu at the end of July. They found 550 unburied Japanese corpses in the town, with more than 100 severely wounded soldiers dying among them. The retreat to the Chindwin became a death march. Japanese forces suffered 54,879 casualties, including 13,376 killed in action -- though thousands more died of starvation, disease, and exhaustion during the withdrawal, a toll never precisely counted. Allied casualties were significant but far lighter. Together with the simultaneous Battle of Kohima, Imphal marked the turning point of the entire Burma campaign. The largest Japanese defeat of the war to that point, it was a catastrophe driven less by Allied tactical brilliance than by Japanese command hubris: a general who believed in his own destiny, supply plans built on captured enemy stores that were never captured, and a military culture that treated retreat as unthinkable even when advance had become impossible. The Imphal plain, once stacked with Allied supplies and ringed by defensive positions, became the place where Japan's ambition to invade India died.