Alagoas participated in the Passage of Humaitá on 19 February 1868 and provided fire support for the army for the rest of the war.

The Pará-class monitors were designed to meet the need of the Brazilian Navy for small, shallow-draft armored ships capable of withstanding heavy fire.

The monitor configuration was chosen as a turreted design did not have the same problems engaging enemy ships and fortifications as did the casemate ironclads already in Brazilian service.

The engines produced a total of 180 indicated horsepower (130 kW) which gave the monitors a maximum speed of 8 knots (15 km/h; 9.2 mph) in calm waters.

[5] Most unusually the gun's Brazilian-designed iron carriage was designed to pivot vertically at the muzzle; this was done to minimize the size of the gunport through which splinters and shells could enter.

She arrived on the Paraná River in January 1868, although her passage further north was barred by the Paraguayan fortifications at Humaitá.

Alagoas and her two sister ships, Rio Grande and Pará were lashed to the larger ironclads in case any engines were disabled by the Paraguayan guns.

Accompanied by Tamandaré, Alagoas bombarded and destroyed the Paraguayan artillery battery at Timbó, upstream of Humaitá, on 23 March.

On 15 October she bombarded Angostura Fort, south of Asunción, in company with Brasil, Silvado, Pará and her sister Ceará.

[11] The article Passage of Humaitá contains contemporaneous descriptions of Alagoas and her sister monitors by captain Richard Burton and colonel George Thompson.