After Argentina intervened in the
Paraguayan War, He attended the battle of Humaitá and the Estero Bellaco, Tuyutí, Boquerón and Sauce battles. He suffered an injury in the hills of Curupaytí. In 1868 he reached the rank of major and later lieutenant colonel and served as military secretary.. Later he was promoted to colonel, thanks to his support for the campaign for the presidency of
Domingo Faustino Sarmiento. By virtue of this, he tried to persuade president to appoint him Minister of War of his cabinet, but Sarmiento did not agree and instead assigned him to the service of the southern border of
Córdoba. Appointed commander of the southern borders of Córdoba, he devoted himself to meticulously exploring the area between the Cuarto and Quinto rivers, as a result drawing a detailed topographic map. Then he made preparations and went into the pampas accompanied by two Franciscan friars and a small escort to deal peacefully with the native people. He visited the chiefs Ramón Cabral and Baigorrita. As a result of this experience, he wrote a series of letters, first published in the Buenos Aires newspaper
La Tribuna as a booklet and shortly thereafter edited in book format, an account that constitutes his best-known literary work,
An excursion to the ranqueles indians . On the return of his expedition, which lasted twenty days, in Villa Mercedes, Mansilla found himself suspended from his post because, proceeding without consulting his boss, he had ordered the execution of a recidivist deserter, after a very brief council of war. President Sarmiento closed the summary making it available, with a warning in his service record. He then devoted himself to journalism, writing articles in the newspapers of the time. Two years later, his friend and president of Argentina,
Nicolás Avellaneda, reinstated him in his military position as chief of staff in Córdoba and later chief of borders and military mayor. From 1882 on he was a deputy during the government of
Julio Argentino Roca, and carried out diplomatic missions abroad. His wife, Catherine, died at the end of 1895. He learned of her death months later, as he was on a mission in Nice. In 1896 he settled in Paris, from where he requested his discharge from the army and two years later he published a biography of his uncle Juan Manuel de Rosas with the character of a historical-psychological essay and then two political essays, "On the eve" (1903) and "A country without citizens" (1907) and sent collaborations for the Buenos Aires press. He was 71 years old when he began to write his "Memories", in which he would recall episodes from his childhood and youth. Towards the end of 1898, on a brief trip to his country, he met Mónica Torromé, widow of Huergo, whose father had installed a commercial firm in London, where she married again the following year and then settled definitively in Paris, in 1902, after carrying out several diplomatic missions in other parts of Europe, functions that he resigned in that year. In his last years he was afflicted with an incipient blindness, and died in that city on October 8, 1913. == Works ==