Battle of Pavón The Battle of Pavón was a key battle of the Argentine civil wars fought in Pavón , in Santa Fé Province , Argentina , on September 17 , 1861 , between the Army of the State of Buenos Aires , commanded by Bartolomé Mitre , and the Army of Republic of the Argentine Confederation commanded by Justo José de Urquiza . The withdrawal of Urquiza left the field to Mitre . It led to the dissolution of the national government and the reincorporation of Buenos Aires Province into the Argentine Republic as a dominant member of the nation . Governor Bartolomé Mitre would act as interim President , ratified by the National Congress , and then as the first President of a unified Argentine Republic . Background . Political postures . During most of the 19th Century , Argentine history was defined by the theoretical , political and military confrontation between two postures : One difference between " porteños " and people from the provinces is that the former did not align directly with the two political parties of the time . Unitarians and Federalists existed both in the capital and in the provinces . Even though they were against each other politically , when it came to defend their own local interests , they joined to confront their common enemy ( be it the capital or the provinces , whatever the case may be ) . Since the secession of Buenos Aires Province on 11 September 1852 , on the aftermath of the Battle of Caseros , Argentina was divided between two competing states , Argentine Confederation and the State of Buenos Aires . The Battle of Cepeda ( 1859 ) and the subsequent Pact of San José de Flores of 1860 set the conditions for Buenos Aires to rejoin the Confederation ( which came to be called Argentine Republic since the 1860 reform of the Constitution on Buenos Aires request ) . However , both sides would clash again soon after . Conflicts in the interior . During president Urquiza ' s government , the provinces had been at peace with the notable exception of San Juan Province , where a political crime served as the catalyst for the Battle of Cepeda between Buenos Aires Province and the Confederation . This changed when president Santiago Derqui took office . Elections in Buenos Aires . As a part of the process leading to the reincoporation of the State of Buenos Aires into the Argentine Confederation , established in the Pact of San José de Flores , after the 1859 Battle of Cepeda , Buenos Aires elected provincial deputies to the National Congress . However , the elections were carried out following the electoral laws of the State of Buenos Aires instead of those of the Confederation . The elected deputies were rejected by the National Congress and the Buenos Aires Senators also staged a walkout , in solidarity . President Santiago Derqui issued a decree invalidating the elections in Buenos Aires and established a new date for a rerun . But the Buenos Aires authorities rebelled against the national government and declared the Pact of San José de Flores null . Civil war . The National Congress considered this as an act of sedition , so President Derqui named Entre Ríos ' s general and former president Justo José de Urquiza as the commander in chief of the national army with the task of returning the rebel province to the fold . In Buenos Aires , Governor Bartolomé Mitre took the post of commander in chief of the provincial army . There were several attempts at mediation , from individuals , and foreign governments . All of them failed due to Mitre ' s and Derqui ' s intransigence . Urquiza tried , until the last moment , to preserve the peace and declined to take the initiative against the porteño army as it was the request of his colonels Ricardo López Jordán and Prudencio Arnold . President Derqui organized an army in Córdoba , gathering an heterogeneous group of infantry units . These forces were augmented by Urquiza ' s , with people from Entre Ríos , Corrientes and Santa Fe provinces , plus some " porteño " defectors ; the majority of these forces being cavalry units . In sum , the federalist army had about 17 , 000 men , where 8 , 000 came from the center region and 9 , 000 from Entre Ríos , Corrientes , Buenos Aires and Santa Fe . Mitre ' s army was made of 22 , 000 men and 35 artillery pieces , plus a considerable numeric superiority of arms and artillery and infantry training . [ The British had supplied the artillery pieces and the trained British artillery crews to operate them ] Derqui advanced up to Rosario , where he left the command of the troops in the hands of general Urquiza , while Mitre advanced to the north of Buenos Aires and advanced into Santa Fe province . The battle . The armies clashed by the Pavón creek , ( south of the city of Rosario , in Santa Fe province , about northwest of Buenos Aires . Urquiza formed his troops in a defensive position , forming an extended line due east of the Domingo Palacios ranch . On the wings he formed his cavalry . Arriving at from the ranch , Mitre deployed his infantry , preparing for an assault on the enemy ' s center . But Urquiza ' s artillery started combat , opening great gaps in the porteño infantry , easy targets due to their colorful uniforms . Combat lasted only two hours , during which the federalist left wing under colonel major Juan Saá , with the Santa Fé and renegade porteño troops of Ricardo López Jordán , completely vanquished the porteño First Cavalry , under general and former Uruguayan president Venancio Flores , chasing them past Arroyo del Medio ( a creek forming the border between Buenos Aires and Santa Fe provinces ) . The porteño Second Cavalry , under the command of veteran general Manuel Hornos , offered more resistance ; but it had to retreat , leaving behind most of their heavier weapons and supplies plus many prisoners . The right wing , under general Miguel Galarza steamrolled the small left - wing cavalry of Buenos Aires . The federalist center , instead , composed by untrained militia from the central regions of the country , was forced to retreat by the better trained and equipped porteño infantry battalions . Seeing the center ' s collapse , Urquiza abandoned the field of battle without adding the 4 , 000 men from Entre Ríos that he had maintained in reserve , and marched to Rosario , then followed to San Lorenzo and Las Barrancas . At that point he received information of his cavalry ' s victory but he did not return to the battlefield . Historians have attempted to explain his retreat but there seem to be no satisfactory answer . The most common reason is attributed to Urquiza ' s being ill and another saying that he mistrusted president Derqui and feared treason . Urquiza ' s unexpected decision left the field open to the porteño army , which had retreated to San Nicolás de los Arroyos . Mitre decided then to consolidate his position before marching later on Santa Fe . Consequences . The battles of Cepeda , Caseros and Pavón were possibly some of the armed conflicts with the most significance in Argentine history , by its institutional consequences , as by the realignment of almost every other political actor after each of the battles . After seeing Urquiza ' s inaction , Mitre gathered his troops . Part of the federalist cavalry advanced to Pergamino , occupying the town . After a reaction from the porteño cavalry , the federalists retreated back to Santa Fé , and Mitre started his advance into that province . Several months had passed from the date of the battle . In the following months , the porteño advance was unstoppable . The only federalist army capable of opposing them was Urquiza ' s , but he did not act and almost dismantled it . Seeing the interior being invaded , Derqui resigned and took refuge in Montevideo . A few weeks later vicepresident Pedernera declared the national government dissolved . Starting on that moment , Mitre projected his influence in the whole country : all the federal governors — with the notable exception of Urquiza — were deposed in the final weeks of the year and the first few weeks of 1862 . Some were deposed by local unitarians , counting on the vicinity of the Buenos Aires army , others directly by the invading porteño army . The ones that avoided that fate , came together to accept that the national government was over , and left to Buenos Aires governor Bartolomé Mitre the task of the national reorganization . Mitre was elected President of the nation by means of new elections - organized by the new provincial governors - from where federalist candidates were forbidden . Porteños also took the national government ministries and a good deal of the seats in Congress . The country ' s capital , which had been relocated to Paraná by Urquiza , was again moved to Buenos Aires city , so the national government had to accept being a guest of the Buenos Aires city government . The location of the new national capital allowed the porteños to defend their interests effectively . In the following years , Argentina maintained a nominal federal organization , but the strength and preponderance of Buenos Aires was unbroken .