 The allies invaded Paraguay in April 1866 , but after initial success , their advance was blocked by fortifications at Humaitá and along the Paraguay River . Porto Alegre had remained behind , leading the 2nd Corps and guarding Rio Grande do Sul . Mitre , no longer on Brazilian territory and acknowledged as the allied commander @-@ in @-@ chief , planned to use Porto Alegre 's army to march through Paraguayan territory from the rear to surround Humaitá . As a result of a suggestion by Tamandaré — though Mitre had not issued orders for the move — Porto Alegre boarded his army onto Brazilian ships and instead brought them up to the positions occupied by the other allied troops . 
