The geographical position of the Argentine made her attitude of decisive importance to both belligerents. Uruguay and the southern provinces of Brazil were separated from Paraguay by the Argentine provinces of Corrientes and the Missions. Argentina had favoured Flores's pretensions, and Lopez was so obnoxious that the secret sympathies of Buenos Aires were with Brazil. Further than neutrality, Mitre, then president of Argentina, would not go. He declared that no permission would be given either belligerent to cross Argentine territory with troops. Lopez was made desperately angry at this refusal; he thought he could count on the alliance and support of Urquiza, the virtually independent ruler of the province of Entre Rios and Mitre's enemy, and seems to have believed that he might as well finish up with both Argentina and Brazil at one sitting. In March, 1865, he deliberately declared war on the Argentine, and eighteen thousand Paraguayan troops crossed the Parana and began offensive operations against Corrientes, Uruguay, and Brazil. Instead of rising against Mitre, Urquiza declared himself against the Paraguayan dictator, and as his province of Entre Rios controlled access to Paraguay by water, Lopez found that the only result of his rash act was to open up the way by which his enemies could most conveniently reach him. On the first of May, 1865, a formal alliance was made between Brazil, Argentina, and Uruguay. Mitre was agreed upon as commander-in-chief; the allies promised not to lay down their arms until Lopez should be overthrown and expelled from Paraguay; and pledges were given to respect Paraguay's independence. Of the three allies Brazil was the only one which could be expected to give its whole force. Flores could only answer for the colorado faction of Uruguay. Argentina did not represent much more than Buenos Aires. Entre Rios was Urquiza's, and the other outside provinces had no great interest in the result. Nevertheless, the alliance was very advantageous to Brazil. It would have been well-nigh impossible to wage a successful war against an enemy shut up in the middle of the continent, and accessible only by a three-months' march across nearly impassable country, or by tedious navigation up a single river running through a third country, and where an army would have to be disembarked direct from ships on the enemy's soil. The adhesion of Argentina made an aggressive war possible, and the event proved how hopeless would have been a campaign by Brazil alone.