Nevertheless, Lopez did not recall the twelve thousand men he had sent across the missions to invade the valley of the Upper Uruguay and the state of Rio Grande. The Brazilians were taken unprepared, and early in August the Paraguayans had captured the chief Brazilian town in that region--Uruguayana. The failure of the Corrientes army to reach the Lower Uruguay left the route up that river free. The Brazilian and Uruguayan army, which had been victorious at Paysandu, marched up the west bank and defeated and destroyed the rear-guard which the Paraguayans had left on the Argentine side opposite Uruguayana. Lopez's army was therefore cut off from retreat. It was promptly surrounded, and on the 17th of September, 1865, had to surrender. This put an end to Lopez's plan of an offensive campaign. Indignant at the invasion of her soil, Argentina had allied herself with Brazil against him. A secret treaty was signed between Brazil, Argentina, and Flores, now recognised as ruler of Uruguay, to prosecute the war to a finish, to depose Lopez from his throne, and to disarm the Paraguayan fortifications. Lopez withdrew his army from Corrientes and concentrated all his forces in the south-west angle of his own territory. The position was admirable for defence. North of the Parana and east of the Paraguay stretched a low, wooded country subject to overflow, and intersected by shallow, mud-bottomed lagoons, which were old abandoned beds of the rivers. The Paraguay protected his right flank and afforded him a direct and easy communication with Asuncion. Batteries on the point of Humaita, which the Brazilian fleet did not dare to try to pass, insured this line of communication. West of the Paraguay the great Chaco, there impenetrable, prevented a movement to get north of Humaita on that side. To the east the swamps along the Parana extended indefinitely, and an advance of the enemy in that direction would have had its communications cut by an army encamped near Humaita. Humaita was, therefore, the key to the situation, and the allies could not advance until they captured it or, by running the batteries with their fleet, destroyed Lopez's control of the Paraguay.