The theatre of operations was quadrilateral, with sides from forty to sixty miles in length, having a fortress at each angle, namely, Bayonne, San Jean Pied de Port, San Sebastian and Pampeluna, all in possession of the French. The interior, broken and tormented by peaked mountains, narrow craggy passes, deep watercourses, dreadful precipices and forests, appeared a wilderness which no military combinations could embrace. The great spinal ridge of the Pyrenees furnished a clue to the labyrinth. Running diagonally across the quadrilateral, it entirely separated Bayonne, St. Jean Pied de Port and San Sebastian from Pampeluna, and the troops blockading the latter were thus cut off from those besieging San Sebastian, the only direct communication between them being a great road running behind the mountains from Tolosa, by Irurzun, to Pampeluna. A secondary range of mountains on the French side of the Great Spine, inclosing the valley of Bastan and lining that of the Bidassoa, furnished positions for the centre and left of the covering armies, with interior but difficult lateral communications. The troops covering Pampeluna were on the Great Spine of the Pyrenees. Behind them were valleys into which the passes across the spine led, descending at the other side in parallel lines, and giving to each division means for a concentric retreat on Pampeluna. Wellington having his battering-train and stores about San Sebastian, which was nearer and more accessible to the enemy than Pampeluna, made his army lean towards that side. His left wing, including the army of siege, was twenty-one thousand, with singularly strong positions of defence; his centre, twenty-four thousand strong, could in two marches unite with the left to cover the siege or fall upon the flanks of an enemy advancing by the high road of Irun; but three days or more were required by those troops to concentrate for the security of the blockade of Pampeluna on the right. Soult thought no decisive result would attend a direct movement upon San Sebastian, and by his seaboard intercourse he knew that place was not in extremity; but he had no communication with Pampeluna, and feared its fall. Wherefore he resolved rapidly to concentrate on his left by means of the great French roads leading to St. Jean Pied de Port, covering his movement by the Nivelle and Nive rivers, and by the positions of his centre: thus he hoped to gather on Wellington’s right quicker than that general could gather to oppose him, and, compensating by numbers the disadvantage of assailing mountain positions, force a way to Pampeluna.