Governor and general

He would later govern Mendoza Province for a short time. He then fought in the war against Brazil in command of 1,200 cavalry, with reported episodes of valour in the battles of Bacacay and Ituzaingó in February 1827. His forces defeated General Abreu and he was proclaimed General on the field of battle itself.

Coup, war, and resignation

By the time he returned to Buenos Aires, Bernardino Rivadavia, the Unitarian Party President of the United Provinces, had resigned, and Manuel Dorrego was elected the federal governor of Buenos Aires Province. Lavalle, a Unitarian himself, led a coup to take the government and executed governor Dorrego, his former leader, without a trial. His government then started a reign of terror, aiming to destroy the Federal Party, but there was resistance in the countryside, which did not recede. In 1829, the demographic growth was negative as there were more deaths than births.

During this time, José de San Martín had returned from Europe. While he was in Montevideo, Lavalle offered him the government of Argentina,^{[citation needed]} because of his authority over leaders on both sides. But when he learned about the spiraling factionalist violence, San Martín realised that he would have to choose sides as the only actual way to govern, so he refused and returned instead to self-exile in Europe.

The other provinces did not recognize Lavalle as the legitimate governor, and supported the rosista resistance instead. Lavalle would be defeated a short time later at the Battle of Márquez Bridge by the forces of Juan Manuel de Rosas and Santa Fe governor Estanislao López. López returned to his province, menaced by Unitarian José María Paz, who had taken power in Córdoba. Meanwhile, Rosas kept Lavalle under siege and forced him to resign with the Cañuelas pact. Juan José Viamonte was designated as interim governor, and the legislature that was removed during Lavalle's coup d'état was restored. This legislature would elect Rosas as the governor. Lavalle retired to the Banda Oriental.

Regroup

During the French blockade to the Río de la Plata, Fructuoso Rivera was reluctant to take military actions against Rosas, aware of his strength. Unitarians, who thought that the whole Argentine Confederation would rise against Rosas at the first chance, urged Lavalle to lead the attack, who requested not to share command with Rivera. As a result, they led both their own armies. His imminent attack was backed up by conspiracies in Buenos Aires, which were discovered and aborted by the Mazorca, a group loyal to Rosas. Manuel Vicente Maza and his son were among the conspirators, and were executed as a result. Pedro Castelli also organized an ill-fated uprising against Rosas, and was executed as well.

Rosas did not wait to be attacked and ordered Pascual Echagüe to cross the Paraná river and take the fight to Uruguay, with López. The Uruguayan armies split: Rivera returned to defend Montevideo, and Lavalle moved to Entre Ríos Province. He expected that the local populations would join him against Rosas and increase his forces, but he found severe resistance, so he moved instead to Corrientes Province. Governor Pedro Ferré defeated López, and Rivera defeated Pascual Echagüe, clearing for Lavalle the way to Buenos Aires.

Defeat

At this juncture, however, France had given up its trust on the effectiveness of the blockade, as what was thought it would be an easy and short conflict was turning into a long war, without clear security of a final victory. France began peace negotiations with the Confederation and cut its financial support to Lavalle. He didn't find help at local towns either, and there was widespread desertion among his ranks. Buenos Aires was ready to resist his military attack, but the lack of support forced him to give up and retire from the battlefield, without starting any battle.

Aftermath

Afraid that his body would be desecrated by the Federales, his followers fled to Bolivia carrying Lavalle's decomposing remains with them. Hurrying over the Humahuaca pass, they finally decided to strip the skeleton by boiling it and, after burying the flesh in an unmarked grave, carry the bones, which are today buried at the La Recoleta Cemetery in Buenos Aires.