Riva-Agüero wrote his senior thesis on "History in Peru" (La Historia en el Perú), the earliest work of historiography to be written in Latin America.

Leguía's Minister of the Interior, Juan de Dios Salazar, ordered his arrest, which immediately provoked bitter university protests and demonstrations that resulted in violent political repression by the National Gendarmerie.

After a long journey through Spain, where he did research in several archives and gave lectures in Seville, Riva-Agüero co-founded the moderate Democratic National Party in 1915 and supported the presidential candidature of the Civilista José Pardo y Barreda in the April 1915 election.

When his mother died in 1926, he succeeded her as Marquis of Montealegre de Aulestia and was legally adopted by her maternal aunt Rosa Julia, 4th Marquise of Casa-Dávila, who later inherited him the title and a large fortune.

In exile, Riva-Agüero abandoned his moderate views and came into contact with the works of Catholic rightist authors such as Jacques Bainville and Charles Maurras and soon became a disciple of their ideas.

[6][verify] In his obituary speech to his colleague of his generation José María de la Jara y Ureta [es], he said that the failure of his political group was due to supporting certain "generous abstractions", such as freedom and democracy.

[3] Initially Riva-Agüero gained a strong following for his new endeavour but before long his newfound extremism, combined with his increasingly odd personal behaviour, began to lose him his credibility.

[1] Meanwhile, he started insisting that followers called him the Marquis of Aulestia, a title that had been in the family but had long since fell into disuse and which had little currency in a republic such as Peru, and his generally arrogant demeanour cost him more support.