The backdrop to the battle was desperation. After the rebel defeat at Huaqui in 1811, Upper Peru -- present-day Bolivia -- had fallen back into royalist hands. Belgrano, placed in command of the Army of the North on February 27, 1812, found himself headquartered in Jujuy with demoralized troops and dwindling supplies. On May 25, he raised the new Argentine flag in Jujuy's cathedral, a gesture of defiance as much as patriotism. By August, he knew he could not hold the city. What followed was the Exodo Jujeño: on August 23, Belgrano ordered the entire civilian population to retreat south toward Tucuman, destroying everything of value as they went. When General Pío de Tristan's royalist army entered Jujuy, they found the city empty. Tristan wrote to Peru's viceroy that there was nothing left. The scorched earth bought Belgrano time, but it did not change the Triumvirate's orders. He was still expected to retreat to Cordoba.

Rumors that Belgrano's army was passing through Tucuman on its way to Cordoba sent the city into a panic. The Cabildo bells rang, and the legislature convened in emergency session. Three representatives -- officers Bernabe Araoz and Rudecindo Alvarado, and the priest Pedro Miguel Araoz -- rode out to meet Belgrano and beg him to stand and fight. When he arrived on September 13, he found Juan Ramon Balcarce waiting with 400 volunteer militia, armed with nothing but lances and organized with impressive discipline. Belgrano, historians agree, needed little persuading. He told the legislature he would stay if they provided 1,500 cavalry troops and 20,000 silver pesos. They agreed. The Triumvirate's retreat orders went unanswered. Meanwhile, Tristan's royalist army was struggling south through territory stripped bare by the Exodo, harassed constantly by local irregulars. When Tristan learned on September 23 that the rebels were entrenched and waiting, the battle was already on Belgrano's terms.

The morning of September 24 broke over a battlefield shaped by confusion and improvisation. Lieutenant Gregorio Araoz de Lamadrid set fire to the fields at Los Pocitos, and the smoke and wind threw Tristan's advancing column into disorder. Belgrano, positioned north of the town, pivoted his front to face west, gaining a clear view of the royalist movements. He organized his infantry in three columns under Colonel Jose Superi, Captain Ignacio Warnes, and Captain Carlos Forest, with a reserve under Manuel Dorrego. When the royalist Cotabambas and Abancay battalions launched a bayonet charge, Belgrano answered with Balcarce's cavalry sweeping into Tristan's left flank. The gaucho horsemen, lances leveled and shouting, scattered the royalist cavalry of Tarija so thoroughly that the fleeing riders trampled their own infantry. But then the gauchos broke ranks. Spotting the royalist mule train loaded with coins and precious metals, they abandoned the pursuit to plunder it. Only the regular dragoons under Balcarce stayed in formation. On the opposite side of the field, the royalists were winning -- Superi was briefly captured before his men freed him. At some point during the chaos, a swarm of locusts descended on the battlefield, darkening the air and disorienting soldiers on both sides.

By nightfall, neither commander fully understood what had happened. Belgrano spent the afternoon trying to reassemble his scattered forces. Tristan, having lost his artillery, ammunition, and supply train, marched on the city and threatened to burn it unless the garrison surrendered. Diaz Velez and Dorrego, now holding the city with captured weapons, responded by threatening to execute their prisoners -- including four royalist colonels -- if Tristan struck a match. The standoff lasted the night. By morning, Belgrano had regrouped behind Tristan's position. Colonel Jose Moldes rode out to demand surrender. Tristan refused with a phrase that would echo through Argentine military history: "The King's soldiers do not surrender." Then he retreated toward Salta, pursued by 600 rebel horsemen. The toll told the story of the battle's lopsided outcome: 450 royalists killed and 690 captured, against 80 rebel dead and 200 wounded. The abandoned materiel -- 13 cannons, 358 muskets, 39 wagons, 70 ammunition boxes -- would equip Belgrano's army for the campaign that followed. On October 27, at a thanksgiving mass, Belgrano placed his command baton at the feet of the Virgen de las Mercedes, naming her General of his Army. The historian Bartolome Mitre would later write that the victory earned Belgrano "the glory of having won a battle against all probabilities and against the wishes of his own government." Three days after Buenos Aires learned of the victory, the First Triumvirate was overthrown. The general who had disobeyed had been vindicated.