The plan depended on silence, and the rock betrayed it. As B Company approached the western peak, codenamed 'Fly Half', Corporal Brian Milne stepped on an anti-personnel mine. The blast tore the silence open. The defenders, a platoon of the Argentine 7th Infantry Regiment under Sub-Lieutenant Juan Baldini, came out of their tents firing. Sappers later counted some 1,500 mines laid along these slopes; only the bitter cold had saved the Paras from worse, because most of the mines had frozen and would not detonate. "Otherwise," one corporal remembered, "the final battle for Port Stanley would have been an altogether different story." What followed was three hours of close combat that the planners had hoped to avoid entirely.

On Longdon there was nowhere to manoeuvre, only the rocks, and the fighting collapsed to a few yards. Small groups crawled bunker to bunker. Two privates wormed up to an Argentine position and each posted a grenade through the firing slit before storming it. The defenders fought back with skill; a Marine sniper named Carlos Colemil picked off attackers one by one in the dark until he himself was wounded, listening all the while to the cries of a wounded radar operator nearby. Sub-Lieutenant Baldini was killed standing up to clear a jammed machine gun, shot as he worked. He was nineteen. Across the mountain, men who had been schoolboys a year earlier were killing and dying within arm's reach of one another.

When 4 Platoon was pinned down by a machine gun it could not silence, its officer was wounded and command fell to Sergeant Ian McKay. Recognising that to stay still was to die, he gathered a handful of men, among them a seventeen-year-old, Jason Burt, and charged the position. The fire was ferocious; his companions were hit one after another. McKay went on alone, throwing grenades, until he was killed at the moment he reached the bunker, his body falling onto it. His sacrifice broke the deadlock and let two platoons pull back to safety. For this he was posthumously awarded the Victoria Cross, Britain's highest honour for valour. His widow received it from the Queen that November. Young Jason Burt did not survive the night.

The British had badly underestimated the men they faced. Brigadier Julian Thompson admitted he came close to pulling his Paras off the mountain: "We couldn't believe that these teenagers disguised as soldiers were causing us to suffer many casualties." The Argentine defenders, mostly reservists with a single year of training, counterattacked in the dark, fought hand to hand for hours, and held positions long after they might have fled. One of them, Private Leonardo Rondi, slipped between groups of Paras to carry messages after his radioman was shot, and was decorated for valour by his own army. The respect was mutual and dearly bought. A British corporal recalled their bunker-busting team battering one position again and again, only to hear the Argentine fire start up the moment the shelling stopped.

By the time the firing died and the sun came up over the cold rocks, the toll was grievous. The British lost 23 dead and more than 40 wounded, with further casualties from artillery in the days that followed; it was the heaviest loss of life the British suffered in any single action of the war. The Argentines lost 31 dead, around 120 wounded, and some 50 taken prisoner. A Para who walked the battlefield afterward came upon four or five of his own friends sprawled dead in a dead-end gap in the path, cut down rushing through it, and felt "both anger and sadness." Mount Longdon fell, and with it one of the last barriers before Stanley. Today a memorial cross to Sergeant McKay stands among the boulders where he died, looking down toward the town his battalion helped to free.