Geography made this island matter. Martín García sits squarely in the main channel of the Río de la Plata, commanding the approaches to the continent's two great inland highways - the Paraná and the Uruguay rivers. Whoever held it controlled the gateway to a quarter of South America. The Spanish navigator Juan Díaz de Solís charted it in 1516 and named it for a storekeeper of his expedition who died aboard and was buried on its shore. For the next three centuries, Spain and Portugal contested the rock. The Viceroyalty of the Río de la Plata fortified it; in 1814, Admiral William Brown landed and broke the royalist hold, handing the island to the young United Provinces. A fort named Constitución, raised in the 1820s, held off the Brazilian navy during the Battle of Juncal in February 1827. Garibaldi himself fought over it. Battle after battle washed across this small island, all for the channel it guarded.

Long before generals exiled their rivals here, Martín García was a place of confinement for the powerless. It served as a penal colony from 1765 until 1886. Then, beginning in 1879, the island became something far darker. As the Argentine state waged the so-called Conquest of the Desert against the indigenous peoples of the south, thousands of captured Mapuche, Tehuelche, and others were shipped to Martín García. Official records show these prisoners of war included not only fighters but noncombatants - elderly people and young children. Many were forced to labor and held under harsh discipline; families were broken apart, with women and children sent into domestic service on the mainland. Researchers estimate as many as 3,000 indigenous prisoners passed through the island in the 1870s and 1880s, and some scholars now describe what happened here as a concentration camp and an instrument of genocide. The herons still wade the same shallows these prisoners once looked out upon.

The twentieth century brought the island a quieter, stranger kind of fame, but the wars of the wider world still reached it. In 1914, German sailors rescued in the South Atlantic at the start of the First World War were interned here for its duration. In 1943, German crewmen from the pocket battleship Admiral Graf Spee - scuttled four years earlier off Montevideo, just across the estuary - were also held on Martín García. An airstrip went in during the late 1920s, grew into a naval air station, and survives today as the island's airport. President Domingo Sarmiento once dreamed bigger: in 1850 he proposed making the island the capital of a new federation uniting Argentina, Uruguay, and Paraguay. The grand plan came to nothing, but it captured something true about the place - a small rock that powerful men kept imagining as the center of everything.

Argentina and Uruguay disputed the island for years, until the Treaty of the Río de la Plata settled the matter on 19 November 1973: Martín García would be Argentine, but solely as a nature reserve. In exchange, Argentina ceded other waters and islands to Uruguay. Then the river redrew the map on its own. Just to the north, a sandbank named Timoteo Domínguez had risen from the current in the 1960s. By the 1980s the silt had built up so completely that the Uruguayan sandbank fused to the Argentine island. The seam between them is now the only land border the two countries share - a frontier the river built grain by grain. Today Martín García is a tourist destination, its old fortifications and prison infirmary slowly greening over, red deer moving through forests of ceiba and laurel, more than 250 bird species filling the canopy. Around 150 people still live here, in a place that fought for centuries to be unconquerable and is now, by treaty, simply left alone.