After the war, a period of «National Reconstruction» began which, although of relative calm, did not know the economic reactivation nor the political peace until 1895 with the presidency of Nicolás de Piérola. With the government of Piérola, a pluto-aristocratic policy was materialized with upper and middle classes who lived comfortably under the auspices of the great American capitals and a common people with various shortcomings, against which they complained, mainly in the face of bad working conditions.
The border with Bolivia, they appealed to the arbitration of Argentina in 1909 to delimit the borders, the Argentine award ends up failing in favor of Peru and compromising Bolivian territories that were not within the discussion that Peru intended to reinvindicate, unleashing disputes. The repulsive Bolivian arbitral award caused the rupture of relations with Argentina, while Peru declared that the non-acceptance of the award would be considered a declaration of war. Both countries understood each other directly and in 1909 a treaty was signed whereby Bolivia ceded to Peru definitively about 250,000 km² in a productive zone of cinchona that included the Madre de Dios river basin and Purus in the Amazon.
This time, known as the Aristocratic Republic, ended with the assumption of Augusto Leguía, who remained in power for eleven years – the Oncenio – with a paternalistic policy towards the indigenous, the creation of a momentary bonanza, the manipulation of the legal order and the intimidation of the opposition. El Oncenio de Leguía, ended in 1930 with the popular pronouncement of Luis Miguel Sánchez Cerro, which initiated a period of military governments and the emergence of popular movements -like the American Revolutionary Popular Alliance or the Peruvian Communist Party- in the political arena.
The Prado government had an international success with the victory of the War against Ecuador in 1941. In June of that year, the Ecuadorian army attacked the zone of Zarumilla, on the Peruvian northern border, which sparked the armed conflict. Peru had formed a unit of paratroopers in the area and made use of it in the first combat in the Southern Hemisphere where airborne troops intervened, which produced the capture of Puerto Bolivar on July 31, 1941, the month when military operations ceased. On the Peruvian side the immolation of Lt. CAP José Quiñones Gonzáles is remembered in the air mission against the Ecuadorian batteries in Quebrada Seca. The Peruvian Army occupied part of the Ecuadorian province of El Oro, next to the Pacific Ocean, as well as parts of the province of Loja, Machala, Puerto Bolívar, Huaquillas, Arenillas, Guayaquil and practically surrounded its capital Quito; reaffirmed its control over the eastern Amazonian territories over which Ecuador claimed sovereignty.
It should be noted that there were only 110 dead on the Peruvian side; on the Ecuadorian side there were more than 1000 deaths after the war ended.
In Rio de Janeiro (Brazil) the Protocol of Peace, Friendship and Limits of Rio de Janeiro was signed on January 29, 1942, which marked the centenary border dispute with that nation, Ecuador loses 245,000 km2 of its territory and its resignation to the territories that it requested as Tumbes, Jaén and Maynas, although the problems derived from the border demarcation would still occupy the rest of the 20th century.
At the end of this third militarism there were democratic presidents interrupted first by the Ocherio of Manuel A. Odría and a brief military coup to continue with the presidential succession. Towards the 1950s the rural exodus began, mainly from the mountains to the cities of the coast, in search of better living conditions and education for their children. Gradually, during the 1960s the political crisis became evident, which led in 1968 to the overthrow of President Fernando Belaúnde in the so-called Revolution of the Armed Forces, the seizure of political power by the Armed Forces of Peru to the command of the general Juan Velasco Alvarado with an anti-imperialist message, especially anti-American, and anti-oligarchy.
A regime of statist court was established that promoted several deep reforms of various results. Towards the end of the 1970s, the military government, with all the reforms produced, was confronted with the economic collapse, even though there had been a change of command in the military leadership and the presidency had been assumed by General Francisco Morales Bermúdez in 1975. Despite this, the revolution ended and democracy was restored.101 A new constitution was drafted through a Constituent Assembly in 1979 and elections were held in 1980.