Leaving Sucre in command of the army, which now threatened Cuzco itself, Bolivar returned to Lima to look after his political interests, collect money, and urge the sending of reinforcements from Colombia. La Serna called in all his outlying divisions, while Sucre confidently scattered his forces. He underestimated the strength of the royalists, for to his consternation La Serna suddenly broke out of Cuzco at the head of ten thousand men, and before Sucre could concentrate, his opponent was threatening his rear and manoeuvring to cut him off from his base. Happily, the royalists were compelled to march in a semicircle, and Sucre, by desperate exertions, united his forces and cut along the radius, coming in sight of La Serna just as the latter had succeeded in getting between him and the road to Jauja. Sucre's position was desperate. The valleys to the north were rising in favour of the royalists; a patriot column advancing from that direction to reinforce him was driven back; his provisions and ammunition were beginning to fail. Sucre's army was La Serna's real objective. Even if he could shake off the pursuit, another march to Lima would be as barren of results as Canterac's last descent, and to leave the Colombian army at Guamanga would expose Cuzco and Bolivia to invasion. During three days the opposing armies marched and counter-marched among the ravines on the west bank of the Pampas River, and finally Sucre took the desperate resolution of crossing the deep gorge in which the river runs in order to reach the high grounds on the other side. He managed to get his main body over safely, but the Spaniards fell upon his rearguard, killing four hundred men and capturing one of his two cannon. The two armies were now opposite each other on the high, narrow, and broken plateau which lies between the Eastern and Central Cordilleras, separated only by the gorge of the Pampas. They marched in plain sight of each other, the royalists along the slopes of the Central Cordillera, while the patriots skirted the foothills of the Eastern. Sucre hoped to outrun the enemy and reach the main road to Jauja, but La Serna again outflanked him; he offered battle, but the viceroy had determined to engage under conditions where not a patriot could escape, and by skilful manoeuvres the royal army succeeded in getting into the protection of the eastern range at a point north of Sucre. Irretrievably cut off from the Jauja road, convinced by his previous failures that he could not better his position by any further manoeuvres, the Colombian general resolved again to offer battle, although this time upon a field chosen by La Serna. He ceased marching and allowed the enemy to dispose their forces at will.