By March, 1866, the allies had concentrated a force of forty thousand men just south of the fork of the rivers. About twenty-five thousand were Brazilians, twelve thousand Argentines and three thousand Uruguayans. The Brazilian fleet, numbering eighteen steam gunboats carrying one hundred and twenty-five guns, lay near at hand ready to co-operate. Protected by the fire of the gunboats, the whole allied army had little difficulty in crossing the Parana and establishing itself on Paraguayan soil. Lopez lost heavily in vain attempts to prevent this landing. On May 2nd, a force of Paraguayans surprised the allies a few miles north of the river and badly cut up the vanguard. The allies, however, continued advancing and took a strong position just south of a great lagoon. Here, on the 24th of May, they were attacked by the whole Paraguayan army of twenty-five thousand men, who fought with desperate valour, but at a hopeless disadvantage. A quarter of the Paraguayan soldiers were left dead on the field, and another quarter were badly wounded, while the loss of the allies was half as great. The Paraguayan army was apparently destroyed, but the allies had suffered so severely, and the difficulties of transportation through the swamps were so great, that they did not make the sudden dash upon the trenches at Humaita which might have ended the war. Lopez did his utmost to reorganise his army. Practically the whole male population was impressed into service. The river line of communication to Asuncion, and the strategic railroad thence up into the most fertile and populous interior of the country, enabled him comfortably to command all the resources of the country, both in men and provisions.