On 16 April 1866, the Allied Armies invaded Paraguayan mainland by crossing the Paraná River. López launched counter-attacks, but they were repelled by Gen. Osorio, who took victories in the battles of "Itapirú" and "Isla Cabrita". Yet, the Allied advance was checked in the first major battle of the war, at Estero Bellaco, on 2 May 1866.