La Frontera was the region around the Bío Bío River - and, in its broader sense, the whole territory reaching south to the Toltén, the heartland of the Araucanía. The term took hold during the long generations when this river marked the southern edge of the Captaincy General of Chile, an outpost of the Spanish Empire and later of the Republic of Chile. To the north lay colonial towns, churches, and royal governors. To the south lay the Mapuche, who in their great revolt of 1598 had destroyed the Spanish cities below the river and pushed the colonial world back to the Bío Bío's banks. The river became less a geographic feature than a fact of power: one bank Spanish, the other free.

Unable to advance, the Spanish dug in. Across the 17th century they laced the land between the Bío Bío and the Itata with a chain of forts, raising, losing, and rebuilding them with grim persistence. In 1601 the governor Alonso de Ribera began the work, fortifying the crossings at the mouth of the Bío Bío and the food-producing estancias inland. Names accumulate like a litany of effort and failure: Santa Fe, Yumbel, Nacimiento, Purén, Buena Esperanza de Rere. Some were ambushed and abandoned within months of their founding; others were destroyed and rebuilt on the same ground three or four times across a single century. One garrison, sent to hold the ruined site of Valdivia far to the south, abandoned its post only after a two-year siege left it starving. In 1641, the entire line of forts south of the Bío Bío was given up under a peace agreement. The forts were not a wall so much as a tide line, marking how far the empire could reach and how often it was driven back.

What force could not settle, diplomacy eventually addressed. On 6 January 1641, beside the Quillén River, Spanish and Mapuche leaders met in a great assembly - a parlamento - and concluded the first peace treaty between the two peoples after nearly a century of the Arauco War. The Parliament of Quilín did something extraordinary for a colonial power: it acknowledged Mapuche autonomy south of the Bío Bío. These parliaments were not mere ceremonies. They were genuine negotiations, conducted with formal protocol and gift exchange, between a European crown and an indigenous nation it had failed to subdue. That recognition, renewed at later parliaments, set the terms for trade, passage, and the return of captives. The frontier it confirmed would endure through the rest of the colonial era and for roughly half a century after Chile's independence - one of the longest-standing indigenous borders in the Americas, and a rare case of a Native people holding their sovereignty against Spain by main force.

By the late 18th century, the man steadily reshaping the frontier was an Irish-born engineer in Spanish service, Ambrosio O'Higgins. He built the fort of San Agustín de Mesamávida in 1777, raised Fort Príncipe Carlos on the Duqueco in 1788, and in 1796 re-founded the city of Osorno, ruined since 1602. O'Higgins rose to become Spain's viceroy of Peru - and fathered a son, Bernardo, who would lead Chile to independence from the very empire his father served. Today the region around the old frontier carries the family's name. The forts have crumbled into farmland and forest, but the line they marked - and the people who held it - remain among the defining facts of Chilean history.