Achnacarry Castle had belonged to the Camerons of Lochiel for centuries, the ancestral seat of one of the great Jacobite clans. In 1940, Sir Donald Walter Cameron of Lochiel handed it over to the war effort. The estate became part of the Special Training Centre at Lochailort, until Brigadier Charles Haydon centralised commando training in December 1941. Until then, each commando unit had run its own training, with predictably uneven results. Haydon's solution was a single school in a place the public could not reach. The British government simply closed the Caledonian Canal crossings to non-residents, effectively sealing off Lochaber. Whatever happened inside the perimeter, very few outside knew about it. That was the point.

Lieutenant Colonel Dudley Clarke had pitched the original idea in 1940, modelling the new units loosely on the Boer Commandos he had studied: small, mobile, raiding behind enemy lines to disrupt rather than to hold ground. The training at Achnacarry reflected that vision and then exceeded it. Recruits learned demolition, close-quarter combat, cliff assault, amphibious landings, vehicle operation, survival, orienteering. They forded rivers under live fire. They scaled wet rock walls in winter. Officers trained alongside their men, an unusual practice for the British Army at the time. The realism cost lives. Recruits drowned in toggle-bridge crossings, fell from cliffs, were struck by accidental fire. The training continued. By 1942 the Royal Marines were reorganising entire battalions into commandos, supplemented by Royal Navy and Royal Air Force volunteers.

From 1942 to 1946, more than 25,000 personnel passed through Achnacarry. They came from British infantry regiments, from the Royal Marines, from Allied nations, and from the French, Polish, Dutch, Norwegian and Belgian forces in exile. In 1943, the focus broadened from raiding to conventional warfare as commando units took on larger roles in the coming invasions of Sicily, Italy, Normandy. After the war, the Cabinet judged that Britain did not need a substantial commando capability in peacetime, and the CBTC at Achnacarry was wound up. The Commando Memorial now stands above the road from Spean Bridge, on a rise that every recruit passed on his march to the castle. Three bronze figures gaze toward Ben Nevis. The plinth lists no names. The men it remembers can be counted in the thousands.

Historians of modern warfare often locate the birth of special forces here, in the rain and the asphalt of Achnacarry. The American Rangers trained alongside British commandos in 1942 and carried the model home. The French commandos, the Dutch Korps Mariniers, the Belgian Para-Commando Brigade all trace lineages through this Highland training ground. The Israeli, Indian, and Pakistani special forces drew on the same techniques through colonial-era instructors. The methodology, intense physical preparation combined with realistic, sometimes lethally so, combat simulation, became the global template. Achnacarry Castle itself returned to the Cameron family after the war. The Nissen huts came down. The drill square reverted, slowly, to grass. But on the hillside above the Spean, the memorial still stands.