Naval Campaign of the War of the Pacific

The Atacama Desert was a rough terrain to conquer and occupy for long. It was nearly waterless and had few roads and railroads. From the beginning of the war it became clear that, to seize or defend the local nitrate resources in a difficult desert terrain, control of the sea would be the deciding factor.

In 1879 Bolivia did not possess any ships, but on March 26, 1879, Bolivian President Hilarion Daza Groselle formally offered letters of marque to any ships willing to go to combat for Bolivia. Bolivia had not signed the Paris Declaration Respecting Maritime Law but the United States, Britain and France stood by the treaty and refused to accept the legality of Bolivia's act. Since Bolivia did not have any ports because Chile had occupied them, and because Peru discouraged the use of Letters of Marque, the naval conflict was left to be resolved between Chile and Peru.

The power of the Chilean navy was based on the twin central-battery ironclad frigates, Cochrane and Blanco Encalada. The rest of the fleet was formed by the corvettes O'Higgins, Chacabuco, Abtao, and Esmeralda, the gunboat Magallanes, and the schooner Covadonga.

The Peruvian navy based its power on the broadside ironclad frigate Independencia and the monitor Huáscar. The rest of the fleet was completed by the corvette Unión, the gunboat Pilcomayo, and the coastal monitors Atahualpa and Manco Cápac, purchased from the United States at the end of the Civil War The coastal monitors cannot be classed among the seagoing ships of Perú as they were permanently stationed, one at Callao and the other at Arica. Although both the Chilean and Peruvian ironclads seemed evenly matched, the Chilean ironclads had twice the armor and held a greater range and hitting power.

...
Wikipedia