The Tacna and Arica campaign is known as the stage of the War of the Pacific after the Chilean conquest of the Peruvian department of Tarapacá, ending with Chilean domination of the Moquegua department in southern Peru. During this campaign Bolivia retired from the war after the Battle of Tacna, and Peru lost the port of Arica. Also, Manuel Baquedano assumed command as the new Commander in Chief of the Chilean Army, and the Allied Presidents were thrown out of office and replaced by Nicolas de Pierola in Peru and General Narciso Campero in Bolivia.
Background
Tarapacá Campaign
After Chile secured the sea with the victory at Angamos on October 8, 1879, the Chilean army prepared to assault Tarapacá Province. An amphibious operation was launched at Pisagua on November 2, pushing the Allies inland and obtaining an unshipping point to unload troops and material. Right after Pisagua, two Allied squadrons were slightly outnumbered and crushed at Pampa Germania on November 6, and Col. Emilio Sotomayor held off a 7,400 men Allied attack on November 19 at Dolores dwell, near San Francisco. The string of victories made the Chilean commanders too confident, and with poor information recklessly attacked the outnumbering Peruvian army at Tarapacá on November 27, suffering a heavy defeat. Nevertheless, the Allies withdrew from Tarapacá department, leaving it in Chilean hands.
Chilean situation
Once they had gained the Tarapacá department, the saltpetre trade earnings went directly to the Chilean government, making the war effort much easier by allowing the purchase of supplies needed for the growing army. Chilean President Anibal Pinto sought to own the saltpetre factories, judging it safer to interpose Bolivia between Chile and Peru, and therefore prepared an invasion to Moquegua department in order to conquer Tacna and Arica, with the ultimate aim of ceding them to Bolivia. In the meantime, Gen. Erasmo Escala resigned his commission as Commander in Chief due to constant and bitter arguments with War Minister Rafael Sotomayor, who then replaced him with Gen. Manuel Baquedano, a Peru-Bolivian Confederacy war veteran.
Allied situation
The loss of Tarapacá generated popular discontent in Peru and in Bolivia, resulting in the deposition of Mariano Ignacio Prado and Hilarión Daza, who were replaced by Nicolas de Pierola and General Narciso Campero. Internal disputes arose between the Allies trying to decide who should lead the army. Finally Gen. Campero was appointed as the commander of the Allied forces, which totaled about 11,000 men.
The Allies ceded the initiative to Chile, allowing her to choose the ensuing course of the campaign. With the Chilean domination of the sea so complete after the earlier battles, the Allies had no manoeuvre space for transporting huge contingents from north to south and vice versa. This also created extreme difficulties in terms of reinforcing the artillery, weaponry, and other supplies of war.
Pierola did not mobilize the Second Southern Army in Arequipa to aid Montero at Tacna, and also replaced Peruvian officers with loyal civilians, resulting in a virtual crippling of the army command.
The beginning
Disembark at Ilo and Pacocha
By the end of December 1879, the Chilean army decided to send a division in a scout expedition to Ilo and Pacocha in order to eliminate the Peruvian resistance in the area and to try to take enemy's supplies, with the order of making plans for a possible attack. The Chilean division was put under Lt. Col Arístides Martínez, and was formed by 500 men of the Lautaro Battalion and 12 riders of the Granaderos a Caballo Regiment.
The Peruvian forces had been gathered at Tacna and Arica, so in order to make a successful attack the entire Chilean army to a place near both cities, avoiding crossing the Atacama Desert.
On December 31, the Chilean troops disembarked and quickly took the port of Ilo, isolating Moquegua garrison of 1,300 soldiers and letting Martinez to move freely in the area, fulfilling the task commended. The expedition returned to Ilo the very next day and was re-embarked to Pisagua.
Between February 18 and 25, a twenty ship convoy unshipped 9,500 men divided into three divisions, while another one stood in Pisagua waiting the convoy to return. Simultaneously, the Peruvian port of Arica was put under a naval blockade by the warships Huascar and Magallanes.