José de San Martín took office as Governor-Intendant of Cuyo (based in Mendoza), positioning himself to build a new army and prepare an Andean crossing to open the route to liberate Chile.
Supreme Director Juan Martín de Pueyrredón decreed the force’s official name “Ejército de los Andes” and confirmed San Martín as commander-in-chief, formalizing the military instrument for the Chile campaign.
San Martín concentrated and trained his troops at El Plumerillo near Mendoza, creating a central staging and drill base for the planned crossing and rapid offensive into Chile.
At El Plumerillo, the Army of the Andes solemnly swore the “Flag of the Andes,” a symbolic milestone marking readiness for the liberation campaign.
San Martín’s force began its operational movement from Mendoza, initiating the multi-column plan designed to deceive royalists and achieve strategic surprise in Chile.
The main body of the Army of the Andes set out from El Plumerillo, executing a coordinated mountain passage in multiple columns over different routes to emerge in Chile nearly simultaneously.
A royalist reconnaissance force struck patriots at Picheuta while Las Heras’s column was still in the mountains, one of the first clashes of the campaign and a warning of nearby royalist activity.
Las Heras’s forces attacked the Spanish post at Guardia Vieja, helping secure passage and supplies on the approach into central Chile ahead of the main concentration.
San Martín’s dispersed columns emerged into Chile within a narrow window, a key operational success that preserved surprise and enabled quick concentration for a decisive battle near Santiago.
San Martín’s Army of the Andes defeated royalist forces at Chacabuco, opening the way to Santiago and shifting the Chilean war decisively toward the patriots.
Following Chacabuco, San Martín entered Santiago, consolidating control of the capital area and enabling the establishment of a patriot government.
With central Chile largely reconquered, Bernardo O’Higgins assumed leadership as (interim) Supreme Director, while San Martín remained focused on the wider continental strategy.
Las Heras defeated a royalist night attack near Concepción, supporting the push to secure southern areas after the capture of Santiago.
The patriot attempt to take the royalist stronghold at Talcahuano ended unsuccessfully, highlighting remaining Spanish control in the south despite the earlier victories in central Chile.
On the first anniversary of Chacabuco, Chile held a formal ceremony and pledge of independence, signaling a political break even as major fighting continued.
Osorio’s forces won a night attack at Cancha Rayada, scattering much of the patriot army and creating a short-lived crisis before San Martín reorganized for a decisive showdown.
At Maipú, San Martín and O’Higgins defeated Osorio, effectively ending major Spanish operations against the core of Chile and securing the revolution’s survival.
José de San Martín — preparations, Crossing of the Andes and Chile campaign (1816–1818)
