President Prieto and his adviser, Portales, fear the efforts of Bolivian general Andres de Santa Cruz y Calahumana to unite with Peru against Chile.

These qualms exacerbate animosities toward Peru dating from the colonial period, now intensified by disputes over customs duties and loans.

Chile also wants to become the dominant South American military and commercial power along the Pacific.

Portales induces Congress to declare war on Peru in 1836.

When a Chilean colonel who opposes the war kills Portales in 1837, this act and the suspicion that Peruvians had been involved in the assassination plot inspires an even greater war effort by the government.