In 1865 circumstances forced Mitre into the Paraguayan war. Lopez, the Paraguayan dictator, hated the Buenos Aireans quite as much as he did the Brazilians with whom he was constantly quarrelling, and he was only awaiting a favourable opportunity to vent his dislike on either or both. He counted on the coolness that naturally existed between Urquiza and Mitre to insure him the former's aid. In 1864 Brazil intervened in the affairs of Uruguay by assisting one of the parties in the civil war then raging. Lopez regarded the action of Brazil as endangering the balance of power in the Plate regions. In retaliation he seized the Brazilian province of Matto Grosso, which lay along the Paraguay north of his own territory. Mitre wished to remain neutral, although he had no sympathy with the brutal despot, and had an understanding about Brazil's action in Uruguay which safeguarded the interests of Argentina. Lopez, however, insolently demanded free passage across Argentine territory for the troops which he wished to send against Brazil and Uruguay. Mitre's refusal was followed by a Paraguayan invasion, and national honour required that this violation of territory be resented. Brazil and the Flores faction in Uruguay welcomed the alliance of Argentina. The Paraguayan invasion was repulsed by their combined forces, and the allies advanced up the Parana against Lopez in his own dominions. It was natural that Mitre should be commander-in-chief of the allied armies, although Brazil furnished the bulk of the troops and bore the brunt of the expense. Urquiza disappointed Lopez in refusing to revolt against Buenos Aires, and although he took no great personal interest in the war he co-operated in many ways with Mitre.