Considerations of international politics and English pressure compelled King John to withdraw his troops from Uruguay in the middle of the year 1812, and the Buenos Aires government immediately began to assemble an army on the right bank of the Uruguay. Artigas was still encamped with his Uruguayan forces in the same neighbourhood, and although he held an Argentine commission he was virtually independent. The Argentine army, under the command of Jose Rondeau, who in colonial days had been captain of guerrillas alongside Artigas, advanced against Montevideo, and on the last day of 1812 won the bloody battle of Cerrito, in sight of the city, and shut the Spaniards up within its walls. Artigas followed and assisted in the siege, but he refused to unite his forces with those of Rondeau until his own claims should be recognised and his demands complied with. He assumed a dictatorship and sent delegates to Buenos Aires to advocate the formation of a federal republic, of which Buenos Aires was to be simply one member. Buenos Aires refused to receive his delegates, and civil war broke out. Rondeau adhered to the Buenos Aires interest; and after a year of disputes, in the beginning of January, 1814, Artigas withdrew his own followers from Montevideo, leaving the partisans of Buenos Aires to continue the siege alone. In May the celebrated Irish admiral, William Brown, destroyed the Spanish fleet, which had hitherto dominated the Plate. Montevideo's communications with both land and sea were shut off, and the fortress shortly afterwards surrendered to General Carlos Alvear, the Argentine general who was then commanding the besieging forces. Meanwhile, Artigas had retired to the west, and the gauchos, not only of western Uruguay, but also of Entre Rios, Corrientes, the Missions, and Santa Fe, rallied around his standard. Independent chiefs in these various provinces had been resisting the efforts of Buenos Aires to reduce them to obedience. Artigas was, in a way, recognised as their leader, but only as the greatest among equals. The conflict with the Buenos Aires party went on throughout the year 1814, and the federalists continually gained ground. In January, 1815, Fructuoso Rivera, one of the lieutenants of Artigas, defeated an Argentine force at the battle of Guayabos, and the Buenos Aires Junta was compelled to withdraw its troops from Montevideo. This, however, did not amount to a separation of Uruguay from the Confederation. It only marked a triumph of the provinces in their efforts to prevent Buenos Aires from establishing a centralised government. Artigas had his friends in Entre Rios, Corrientes, the Missions, and Santa Fe, and even as far as Cordoba; and Francia, dictator of Paraguay, was another of his allies in this struggle against Buenos Aires. However, he was nothing more than a military chief, without the capacity or even the desire of uniting these vast territories under a rational and stable government.