The Tarija War (Spanish: Guerra por Tarija), also known as the War between Argentina and the Peru–Bolivian Confederation (Spanish: Guerra entre Argentina y la Confederación Perú-Boliviana), was an armed conflict that occurred between 1837 and 1839. Because it happened while the Peru–Bolivian Confederation was engaged in a parallel war against the Republic of Chile during the so-called War of the Confederation, both conflicts are often confused. The Tarija War began on May 19, 1837, when Juan Manuel de Rosas, who was in charge of managing foreign relations for the Argentine Confederation and was governor of the Province of Buenos Aires, declared war directly on President Andres de Santa Cruz because of the Tarija Question and Confederation's support for the Unitarian Party.

The operations began in August 1837, when Bolivian Confederate troops invaded most of the Province of Jujuy, the Puna de Jujuy, and the north of the Province of Salta. The war continued with a series of combats and skirmishes between both forces, all of them without conclusive results. In May and June 1838, the Confederate army defeated Rosas's troops in a series of engagements, the most important of which was the Battle of Montenegro, which in practice led to Argentina's withdrawal from the war, which from then on had a defensive posture being maintained, but the state of war continued until the victory of the Chilean-Peruvian restorative army at the Battle of Yungay, which put an end to the Peru-Bolivian Confederation.

Relations between the Peru-Bolivian Confederation and the Argentine Confederation had deteriorated, among other reasons due to Bolivian President Andrés de Santa Cruz's support for members of the Unitarian Party that carried out at least four incursions from southern Bolivia to the northern Argentine provinces in the years before the war. One of them was that of Colonel Javier López in 1834, which culminated in his defeat in the Battle of Chiflón. A new attempt by López in 1835 was defeated at the Battle of Monte Grande, where he was shot. Also in 1835 Felipe Figueroa invaded the Province of Catamarca and in 1836 Mariano Vázquez, counting Bolivian forces among his ranks, attacked towns in Puno

In August 1835, Argentine forces entered Bolivian territory to arrest José Antonio Reinafé and Cornelio Moyano, which exacerbated border tensions.

Rosas suspected that Santa Cruz was using the Unitarians, in alliance with Uruguayan President Fructuoso Rivera, to seize the provinces of northwestern Argentina, since it had already done so in Peru. In September 1836, the Chilean confidential agent Francisco Javier Rosales showed Rosas documents in this regard, which confirmed the rumors that the unitary Juan Galo Lavalle was in negotiations with Santa Cruz to establish a state between Argentina and Bolivia.

Persuaded by the minister Diego Portales, José Joaquín Prieto declared war on November 11, 1836, with the northern Peruvians opposed to Santa Cruz and its confederation. Portales communicated the declaration to Rosas and asked him to do the same by virtue of the tacit understanding between the two countries and reiterated the offer to sign an alliance treaty.

On December 28, 1836, Rosas wrote to the governor of Salta Felipe Heredia:

because Bolivians only live on the tribute of the Indians and what the Cerro and Casa de Moneda de Potosí produce (...) and the seizure of that Villa assures me that it is not a company of great difficulty (...) it owes us the millions of pesos that we have spent for their freedom and independence in the war against the Spaniards (…) and they owe us the Argentine blood that has been spilled in this war (…) Understand that, once Tarija has been restored, the Suypacha River will have to divide the territory of both republics; But it seems to me that if we can ensure that the Villa de Tupiza and the town of Santiago de Cotagaita remain within our territory, it will be the best and most important thing to ensure peace and free trade between both States forever, with all the advantages that I have indicated. In exchange for doing this good I think we could forgive the expenses made in the War of Independence and also the benefits that they have taken from Tarija in all the time it has been usurped. But to obtain all these things it will be necessary to penetrate to the Capital of Bolivia, and have the Cerro de Potosí for us. Such an important acquisition must be at work with the exclusion of the Salteños and Jujeños