To understand the raid, look across the border. After Guinea voted for independence in 1958, President Ahmed Sékou Touré opened his country to liberation movements still fighting colonial rule. Chief among them was the PAIGC, the African Party for the Independence of Guinea and Cape Verde, led by the brilliant agronomist-turned-revolutionary Amílcar Cabral. From bases in and around Conakry, the PAIGC waged a guerrilla war against Portuguese rule in neighboring Portuguese Guinea, today's Guinea-Bissau. For Lisbon, Conakry was the sanctuary that kept the insurgency alive. Eliminate Cabral, the thinking went, and the rebellion might collapse. The operation was placed under a young marine officer, Commander Alpoim Calvão, and given the codename Mar Verde, Green Sea.

Roughly 220 Portuguese soldiers, both African and European, came ashore alongside some 200 Guinean exiles who hoped to overthrow Touré. They moved with several aims at once. One group destroyed PAIGC supply vessels in the harbor. Another landed near the president's summer house and burned it. Others struck at the PAIGC headquarters, hunting for Cabral, who, as it happened, was in Europe. A separate party broke into the prison camps and freed a number of captives, including 26 Portuguese prisoners of war held at Camp Boiro, some confined for as long as seven years. But the operation stumbled on a small, decisive error: the raiders attacked a radio station that had been abandoned, never silencing the one still broadcasting under government control.

With the prisoners freed, half the invading force withdrew to the waiting ships, leaving fewer than 150 men to accomplish the hardest task of all, toppling a government. They appear to have gambled on a popular uprising. None came. The functioning radio station rallied the city, most senior officials evaded capture, and with neither Cabral nor Touré in their hands, the raiders had run out of objectives. They retreated to the sea, having suffered only minor casualties. As a military exercise the raid was swift and partly successful. As a coup it failed completely, and the failure mattered most for the Guineans who had stayed behind, and for the thousands who had never left.

What followed was harsher than the battle. Within a week Touré created a ten-member High Command that ruled by decree, ordering arrests, detentions without trial, and executions. After a five-day trial in January 1971, a Supreme Revolutionary Tribunal handed down 29 death sentences carried out three days later, dozens more in absentia, and scores of life sentences at hard labor. Among the dead was the country's finance minister. Many of the accused vanished into the camps. The world condemned the raid: on 8 December 1970 the UN Security Council passed Resolution 290 censuring Portugal, and the Organization of African Unity followed with a unanimous resolution of its own. Yet the invasion's deepest mark was internal. It gave a fearful regime its pretext, and Guinea paid in the years of repression that came after.