Costa du Rels was provided with many pamphlets on coca to help him defend the Bolivian position at the League of Nations.

[10] The arguments in defense included the economic importance of the crop, its value in health and nutrition, and the fact that its use was a long tradition among Bolivians.

[11] In July 1932 an ongoing dispute between Bolivia and Paraguay over the Gran Chaco territory escalated into armed conflict.

In September of that year the League of Nations set up a three-person committee with delegates from Ireland, Spain, and Guatemala to investigate the conflict and if necessary prepare for intervention.

The committee head, Seán Lester, attempted without success to resolve the positions of two countries' delegates to the League, Costa du Rels and Ramón Caballero de Bedoya of Paraguay.

[14] He represented to the League that the territory had belonged to Bolivia since 1810, but that Paraguay had been surreptitiously occupying the region, taking advantage of her favorable geographical position.

[17] Ironically, since the war had been fought over suspected oil and gas reserves, it turned out later that there were none in the Paraguayan sector while the Bolivian quarter was rich in these resources.

[19][6] In July 1940, Costa du Rels supported the appointment of Seán Lester as Secretary-General of the League, a diplomat with whom he had negotiated during the Gran Chaco conflict.

[21] In that position he attempted without success to obtain support from the United States for raising the price paid for Bolivian tin, which had been fixed at a relatively low level but for which there was now a shortage of supply.

[25] His essay Los cruzados de alta mar (Deep sea crossings) won the Prix Rivarol in Paris in 1954.

[27] Some of his stories such as La Misk'isimi (Quechua misk'i sweet, honey, simi mouth, word, language)[28] explored indigenous Bolivian themes.

[29] This story appeared in the French-language 1928 collection La hantise de l'or [The spell of gold], published in 1948 in Spanish as El embrujo del oro.

[30] Costa du Rels was given many awards, including the National Prize for Literature in 1976 and the Grand Cross of the Order of the Condor of the Andes.