At the beginning of the Cruzada Libertadora in 1825 - a revolt against Brazilian domination of the Banda Oriental - the young Melchor, aged 16, joined the rebel troops of Julián Laguna in Mercedes, who appointed him his secretary.

When Rivera was proclaimed president for the second time in February 1839 and declared war on the government of Buenos Aires, Pacheco joined the forces commanded by Rufino Bauzá.

After the battle of Arroyo Grande (December 1842), he left his office and joined Rivera's army, which was returning after being defeated; the president made him Minister of War and Navy.

He lived there for a year with difficulties, but in 1845 he was called by the President of the Gobierno de la Defensa, Joaquín Suárez, who promoted him to general and entrusted him with total command of the besieged troops.

[1] Disagreeing with the situation created by Rivera's reappearance that year, which culminated in a military mutiny, he resigned from his positions on April 2 and took refuge on a French ship that took him back to Rio de Janeiro.

When the unexpected confrontation broke out between the National Guard (mostly from the Blanco party, and it is said, unarmed) and the 2nd Battalion of Hunters Led by the Spanish mercenary León de Pallejas, promoter of the coup, Melchor Pacheco appeared before President Juan Francisco Giró, who was in a meeting with the diplomatic corps, and told him that a riot had broken out that he personally disapproved of, but that he had obligations that he could not ignore with his comrades in arms, whom he was going to join in trying to restore order.

He was one of the main promoters of the constitution of the Triumvirate that succeeded Giró and was awarded the position of Chief of the Army General Staff, which put military power in his hands.

Pacheco y Obes, whom José E. Rodó considered the most fascinating figure in Uruguayan history, was a military man who stood out more as an organizer, speaker and journalist than as a soldier.