Mercado's Federal army, after its dramatic and terrible retreat four hundred miles across the desert when Chihuahua was abandoned, lay three months at Ojinaga on the Rio Grande. At Presidio, on the American side of the river, one could climb to the flat mud roof of the Post Office and look across the mile or so of low scrub growing in the sand to the shallow, yellow stream; and beyond to the low mesa, where the town was, sticking sharply up out of a scorched desert, ringed round with bare, savage mountains.