In October Bolivar issued a proclamation foreshadowing the union of Venezuela and New Granada. In the midst of defeat, with all of both countries except the thinly populated Orinoco plains in possession of the Spaniards, he was confidently planning the creation of a great empire. Morillo opened the campaign of 1819 by advancing with over six thousand men against Paez on the upper Orinoco. The Creole's four thousand were mostly cavalry, and he had learned better than to risk a pitched battle. The Spanish columns were harassed beyond endurance by his light horsemen, and after weeks of heartbreaking marches Morillo had to retire, having accomplished nothing. From Bolivar's erratic genius now emanated a great stroke of strategy. West of the plains of the Apure and Casanare, tributaries of the upper Orinoco, rises the giant range of the Cordillera and on its top lay the fertile plateaux of Socorro, Tunja, and Bogota, the populous heart of New Granada. For three years the Spaniards had been in secure possession and all except three thousand troops had been drafted for service in Venezuela and Peru. A small Spanish force came down from Tunja to attack the patriot guerillas in Casanare, and was repulsed. Where the enemy could go he could follow, reasoned Bolivar. Paez's cavalry had proved itself amply able to hold the llanos, so no risk to Venezuela would be incurred by temporarily withdrawing part of the infantry. With two thousand natives and five hundred British the Liberator followed up the Orinoco, Meta, and Casanare to the latter's sources at the foot of the Paya pass, which leads directly into the fertile valley of Sagamoso, the heart of Tunja province. This pass is high and very difficult, although the distance to be traversed was only eighty miles. The road was a mere track leading along precipices, crossing and recrossing mountain torrents, and the rain fell incessantly as the patriots struggled up the slippery path. When they reached the higher regions a hundred men perished with the cold, and not a horse survived. The army arrived at Sagamoso in a pitiable condition, but without seeing an enemy except an outpost, which was easily dislodged.