[1] In 1878, he continued his higher studies by entering the Faculty of Law of the Universidad Mayor de San Andrés (UMSA), but due to the occupation by the Chilean army of the Bolivian town of Antofagasta on 14 February 1879, Montes decided to leave his studies and enlist as a private in the Murillo Regiment, then belonging to the "Bolivian Legion".

[citation needed] Upon his return to Bolivia, due to his heroism during the battle, Montes was directly promoted to the rank of captain by the government.

[1][2] In 1890, at the age of twenty-nine, Montes was elected as a Deputy representing the Liberal Party (Bolivia), however, his ideology collided with the prevailing conservatism of the time.

[citation needed] As a deputy, Montes was known for his elegant and eloquent personality, making him a perfect partner to the vociferous and mercurial Atanasio de Urioste Velasco, another staunch liberal of the time.

[2] Conservative President Severo Fernández wanted to settle the decade-long debate regarding what city was officially the Bolivian capital.

The liberals had done so strategically since if they had vetoed it they would have provoked the inhabitants of the capital, and they knew that if it was approved they could convince the people and the garrison of La Paz (under the orders of Colonel José Manuel Pando) to mount an insurrection.

In response, on 12 December, with the people of La Paz behind them, a Federal Board of Liberals was formed, which included some authority figures who had switched sides (these being the Prefect and Commander General Serapio Reyes Ortiz and the Minister of Instruction Macario Pinilla).

During the entire duration of the conflict, Montes remained a loyal partisan to the liberal cause and fought under Pando's command.

[1] During his time as minister, Montes was concerned with improving the army, subjecting it to greater discipline and equipping it with modern material.

[2] Montes also led a military expedition to fight in the north of the country against Brazilian filibusters in the so-called Acre War (1900–1903).

[12] According to Bolivian historiography of the mid-20th century onwards, this treaty was the result of the harassing pressure exerted by Chile on Bolivia (motivated by the expropriation of Chilean and foreign capital that triggered the War of the Pacific), with customs controls and trade restrictions.

[13] The liberal governments of Pando and Montes believed that it was time to turn the page with Chile and were convinced that the development of the railways and free transit, stipulated in the treaty, were compensations that were worth the sacrifice.

[2][3] A staunch liberal, Montes established civil marriage, freedom of worship and the abolition of ecclesiastical jurisdiction as fundamental liberties and rights in the Bolivian Constitution.

[1] This caused a rupture between the Holy See and the Bolivian government, prompting Pope Pius X to issue the apostolic letter Afflictum propioribus in November 1906.

As the end of his constitutional period approached, Montes promoted the candidacy of the liberal José Gutiérrez Guerra, a childhood friend of his most loyal ally, Atanasio de Urioste.

While he was still serving as military advisor, due to his advanced age, he suddenly died on 16 October 1933, in the city of La Paz.