It would have been well if the energies of Mitre had been left unhampered to settle the thorny question of the respective claims of the Porteños and the provincials, but it was the misfortune of Argentina to be suddenly involved in the most serious foreign war of its history. This was the great Paraguayan war.
The occasion of the hostilities was Uruguay. That country had long been distracted by the savage political strife of the Colorados and Blancos, and in 1864 the Blancos, having got the upper hand, elected Dr. Aguirre to the Presidency, who, by his rigorous measures against all suspected of disaffection, excited the resentment of both Argentina and Brazil. Both of these countries had important stock-raising interests on the Uruguay frontiers, and in the civil turmoil their subjects were frequently subjected to extortion and plunder. Having incurred the hostility of its too-powerful neighbours, Uruguay looked about for an ally, and found one in General Lopez, the Dictator of Paraguay.
Lopez in his youth had visited Europe, admired the great armies of the Continent, and returned convinced that he might play the part of Napoleon in South America. He had still hardly reached middle age and was able, cruel, and obstinate. He devoted all his efforts to raising and equipping an army by which he hoped to make himself the arbiter of South American politics. Accordingly he welcomed the appeal of Uruguay, and declared that he would regard an invasion of Uruguay by Brazil as an unfriendly act. When Brazil attacked Uruguay he did not, indeed, hasten to fulfil his promise; he cared nothing for the Colorados or Blancos, and the difficulties of invading Brazil were at first insuperable, but he was awaiting a favourable opportunity for his own aggrandisement. As far as Uruguay was concerned the Brazilians soon settled the matter; they beat down all resistance, set up Flores as President in February, 1865, and having established good relations with Montevideo, withdrew their army. But Lopez was a more difficult problem.
Lopez had already declared war; he had attacked Brazilian ships and made preparations to invade Rio Grande do Sul. His main object was to crush the Brazilian troops in the Plate district before they could be reinforced. His plans were bold, but there appeared no reason why they should not be successful. He had forty-five thousand infantry, ten thousand cavalry, and adequate artillery. Another fact in his favour was the friendship of Urquiza, now Governor of Corrientes, who was the enemy of Mitre. Both Brazil and Paraguay requested permission from Mitre to march their army through Misiones, but the President wished to remain neutral and refused both requests. Lopez, however, was dismayed by no obstacle, and directed General Robles with twenty-five thousand men to invade Corrientes. They soon overran the province. Argentina was in an awkward position, for her regular army amounted to six thousand men only, but she had the support of Brazil and Uruguay. On June 2, 1865, the defeat of the Paraguayan fleet by Brazil at Riachuelo baulked Lopez's schemes of an offensive war, and the allies prepared to invade Corrientes. Lopez was further hampered by the defection of Urquiza, who finally refused to assist him.
The plan for the invasion of Rio Grande had not been abandoned, but the Paraguayan force was opposed by that of Brazil and Uruguay on August 17th, and suffered defeat. A month later the defeated army under Ertigarribia surrendered. Before the end of the year Lopez was compelled to evacuate the Argentine territory.
But his position was less unfavourable than might be supposed upon a comparison of the resources of the contending countries, for he had an excellent army and the country between the Parana and the Paraguay was admirably adapted to defensive operations. It was densely wooded and liable to floods which often made it impassable. As long as he could hold Humaita, where he had erected batteries to stop the Brazilian fleet, it would be impossible for the allies to make an effective advance. They had an army of forty thousand men concentrated near the town of Corrientes, and by April, 1866, they had forced the passage of the Parana and were in Paraguay. On May 24th they were attacked by twenty-five thousand Paraguayans and a desperate battle ensued, which ended in the victory of the allies. The Paraguayans lost five thousand killed and wounded, and their opponents, who lost about two thousand, were so severely crippled that they could not advance upon Humaita—a movement which might have ended the war.
The army was devoted to Lopez and the Paraguayan made a fine soldier, while the allies, encamped in unhealthy swamps, lost heavily from disease. Mitre at last began an onward movement, but on September 22nd he was repulsed with great slaughter at Curupaiti, and the war came to a standstill. There was a long pause, for the difficulties of the allies were enormous and cholera broke out in their camp. It was not till November, 1867, that the Brazilian army succeeded in crossing the Paraguay north of Humaita, and this clever movement of the Brazilian Marshal Caxias decided the fate of the war. The allied armies began to close round Humaita and Brazilian warships forced their way beyond Curupaiti. Lopez fought with remarkable determination and skill, but his embarrassments rapidly increased, and on February 18, 1868, the Brazilian fleet ran the batteries at Humaita, and this entirely disorganised the transport system of Lopez, who relied chiefly upon his waterways. All through the year fighting continued, but on December 27, 1868, Lopez received a crushing defeat at Angostura, south of Asuncion, and he was compelled to fly into the interior. A few days later the Brazilians occupied Asuncion. But the irrepressible Lopez proclaimed a new capital at Peribebuy and made desperate efforts to carry on the war. After much fighting the allies succeeded in capturing the town in August, 1869, and pursuing Lopez, defeated him at Campo Grande, the last pitched battle of the war. He fled into the forest with his mistress, Madame Lynch, his children, and numerous faithful followers. Even in this extremity he still kept the field, until at last, on March 1, 1870, while he was encamped far to the north on the river Aquidaban, his men were thrown into a panic by the approach of the enemy. In the confusion Lopez and his staff attempted to escape, but the General's horse stuck in a swamp; he refused to surrender, and was killed by a spear-thrust. Thus died this extraordinary man, who had wantonly led his country into a war in which five-sixths of her population perished.
During this long war the domestic history of Argentina was uneventful. Brazil was far more prominent in the war than Argentina, for General Mitre was several times distracted by rebellions in the north-west which called him from Paraguay. The rebels were easily driven across the Andes. But the constitutional question had never been settled, and hostility to the Porteños became stronger. The influence of Mitre had waned, and in 1868 Sarmiento was quietly elected in his place.
The close of the Paraguayan war is also the close of the anarchical period of Argentina's history. Hereafter, though she was often to be unwisely governed, the worst of the wars and revolutions were at an end, and the people were to devote themselves to developing the natural wealth of the country. Since the Revolution, her history had been almost as bloodstained and turbulent as the worst of her neighbours, but henceforward peace and prosperity, though not uninterrupted, are to distinguish her from the other South American Republics.