Immediately after the victory of Tucumán, obtained by Belgrano against the Spanish forces that invaded northern Argentina after the defeat of Huaquí, the siege of Montevideo was begun once more, both as a result of that victory and of negotiations with Brazil, whereby the troops of that empire, which had come to the aid of the Spaniards, were to withdraw from Uruguay. The patriots were soon able, in spite of the friction that arose between Artigas and the Argentine troops, to pin the Spanish forces within the inner circle of the defenses of Montevideo. It was in connection with this campaign against Montevideo that San Martín in 1813 won his first victory against the Spaniards, at San Lorenzo, on the shores of the Paraná. By this victory an end was put to the foraging expeditions that the beleaguered garrison of Montevideo sent to the province of Entre Ríos. Thus the only immediate avenue for the supplying of provisions was closed. The desertion, however, of Artigas, and the harassing by his Uruguayan bands of the besieging forces, threatened once more to bring about the abandonment of the siege. The defeat of the naval forces of Spain in the Plata River by Admiral Brown, an Irish sailor in the service of the government of Buenos Aires, came at this time most opportunely to close all avenues of replenishing, and the garrison was forced to capitulate. The taking of Montevideo was an event of the utmost importance for the Revolution, since it made possible the concentrating of all forces for an attack upon Peru, the heart of Spanish power in South America.
Undeterred by the disastrous results of the first Army of the North at Huaquí, the provisional government decided upon a second expedition to Alto Perú. In addition to the strategic difficulties of this plan, new obstacles arose from the jealousies and ill-feeling caused by the ambition of General Alvear. He has truly been called the Alcibiades of Argentina. Handsome, young, accomplished, and erratically brilliant in the conception of his military plans, he was consumed by a disproportionate ambition to be the leading military hero of the Revolution. It was in order to satisfy this inordinate craving for glory that he had himself appointed general-in-chief of the besieging forces of Montevideo, at a moment when that city was doomed to fall, thus depriving Rondeau of a victory which in reality was his. As in the case of the campaign of Montevideo, Rondeau was first selected to command the second Army of the North, but Alvear intended to keep him in command only until the time should come for decisive action, when he himself, aided by the ascendancy he had gained with the provisional government, would assume command and reap the fruits of victory. In accordance with this plan, he attempted to supplant Rondeau towards the end of the year 1814. This time, however, Rondeau refused to deliver his command to Alvear, and, as his troops supported him, Alvear was forced to return to Buenos Aires. The great popularity that he enjoyed in the capital raised him to the supreme honor of Director of the United Provinces, but his harshness in dealing with opposing political parties, and his weakness in meeting the rebellion of Artigas, combined with the opposition of both the Army of the North and the army that San Martín was quietly getting together in Mendoza for the invasion of Chile, made his tenure of office very short-lived, and he was forced to surrender the reins of government and flee to Río Janeiro. This was the first serious defection among the makers of the Revolution, if we except the work of the Uruguayan leader, Artigas, who by his constant opposition to the provisional government of Buenos Aires delayed and prevented the unification of forces necessary to bring about the fall of Spanish power, but who at least worked for the independence of Uruguay, whereas Alvear sought primarily to aggrandize himself. It was not surprising therefore that the Army of the North, its discipline shaken by the recent rebellion caused by Alvear’s ambition, and confronted, as it was, by the naturally difficult task of attacking Peru by a road which had been marked by the previous defeat of Huaquí, met a second disastrous defeat at Sipe-Sipe. So complete was this defeat that the Army of the North ceased to exist entirely, never to be reorganized again as it had been after Huaquí. What Belgrano did for the defense of the provinces after the defeat of the first Army of the North, the caudillo Güemes accomplished after Sipe-Sipe. With his swift band of gauchos he conducted such an effective guerrilla warfare against the victorious Spanish army, that they were unable to derive the full benefit of their great victory.
The fortunes of the Revolution had never sunk so low as after the battle of Sipe-Sipe. Yet, it was immediately after this disaster that the Congress of Tucumán, on the 9th of July, 1816, proclaimed officially the independence of the United Provinces of the Plata. This was an artificial way of reviving the hopes of the Revolution, for what was wanted then was action, not words. Withal it did serve to rouse the failing enthusiasm of the people. Moreover, to substantiate the rhetorical enthusiasm of the Declaration of Independence, there soon came from an unexpected quarter deeds of such magnitude as to destroy for all time the power of Spain in America.