Reille called in Maucune from Frias, and having fears for his safety gave him a choice between a direct road across the hills, or the circuitous route of Puente Lara. Maucune started late in the night of the 17th by the direct road; and meanwhile Reille having reached Osma on the morning of the 18th, found a strong English column issuing from the defiles in his front, and in possession of the high road to Orduña. This was Graham. He had three divisions and a considerable body of cavalry, and the French general, who had eight thousand infantry and fourteen guns, engaged him with a sharp skirmish and cannonade, wherein fifty men fell on the side of the allies, above a hundred on that of the enemy; but at half-past two o’clock, Maucune had not arrived, and beyond the mountains, on the left of the French, the sound of a battle arose and seemed to advance along the valley of Boveda in rear of Osma. Reille, suspecting the truth, instantly retired fighting towards Espejo, where the mouths of the two valleys opened on each other, and then suddenly, from that of Boveda Maucune’s troops rushed forth, begrimed with dust and powder, breathless and broken.
That general had, as before said, marched over the Araçena ridge instead of going by the Puente Lara, and his leading brigade, after clearing the defiles, halted near the village of San Millan in the valley of Boveda, without planting picquets; he was there awaiting his other brigade and the baggage, when suddenly the light division, moving on a line parallel with Graham’s march, appeared on some rising ground in front. The surprise was equal on both sides, but the British riflemen instantly dashed down the hill with loud cries and a bickering fire, the 52nd followed in support, and the French retreated fighting as they best could. The rest of the English regiments remained in reserve, thinking all their enemies before them, but then the second French brigade, followed by the baggage, came hastily out from a narrow cleft in some perpendicular rocks on their right hand, and a confused action ensued. For the reserve scrambled over rough intervening ground to attack this new foe, who made for a hill a little way in front, and then the 52nd, whose rear was thus menaced, quitting their first enemies, wheeled round and running full speed up the hill met them on the summit; so pressed, the French cast off their packs, and half flying, half fighting, escaped along the side of the mountains, while their first brigade, still retreating on the road towards Espejo, were pursued by the riflemen. Meanwhile the sumpter animals, sadly affrighted, run about the rocks with a wonderful clamour; and though the escort, huddled together, fought desperately, all the baggage became the spoil of the victors, and four hundred of the French fell or were taken: the rest with unyielding resolution and activity escaped, though pursued through the mountains by some Spanish irregulars: Reille then retreated behind Salinas de Añara.