As the Brazilian army was ready for combat, Caxias sought to encircle Humaitá and force its capitulation by siege. To aid the operation, he used observation balloons to gather information of the enemy lines. The combined Brazilian–Argentine–Uruguayan army advanced through hostile territory to surround Humaitá. By 2 November, Humaitá was completely cut off from land reinforcement by Paraguayan forces. On 19 February 1868, Brazilian ironclads successfully made a passage up the Paraguay River under heavy fire, gaining full control of the river and isolating Humaitá from resupply by water.