When summer came in 1817, and all the passes were freed from snow, he was ready. In the middle of January he broke camp at Mendoza and divided his army into two divisions. Directly to the west was the Uspallata Pass, then as now the usual route between western Argentina and central Chile. Its Chilean outlet opens into the plain of Aconcagua, which is north of Santiago and only separated from that capital by one transverse spur of the Andes. Off to the north was the more difficult pass of Patos, its eastern entrance also easily accessible from Mendoza, though by a longer detour, and opening at its other end into the same valley of Aconcagua. The smaller of the two divisions was to advance over the Uspallata Pass, so timing its movements as to reach the open ground of the Aconcagua valley at the same time as the larger division, which, under San Martin himself, went to the north around the Patos route. The Spaniards had a guard at the summit of the Uspallata Pass, but the advance troops of the Argentines charged it. Before re-enforcements could come up, the division was over and advancing confidently down the canon on the Chilean side. Had the Spaniards sent up a force sufficient to prevent the Uspallata division from debouching on to the Aconcagua plain it would have been caught in a trap. The second division could have bottled it up from below by leaving a small body at the mouth of the canon. But before the Spanish commander had made up his mind what to do, news came that another army was rapidly coming down the valley leading into the Aconcagua valley from the north. Disconcerted by this attack from an unexpected direction, the Spanish commander hastened off with an inadequate force to repel it. He did not reach a defensible point in time; his vanguard was defeated and he retreated along the highroad to Santiago, leaving San Martin to reunite his two divisions at his leisure in the broad Aconcagua plain. Though the army had crossed the Andes over two of the loftiest and steepest passes in the world, so admirably had all dispositions been made that hardly a stop was necessary to refit and recruit. Artillery and cavalry, as well as infantry, were ready within four days after reaching the Chilean side to take up the pursuit of the Spaniards.