This town lies twenty miles to the north of the Ebro on the right bank of one of its tributaries, the Sicoris (Segre), which was crossed by only a single solid bridge immediately at Ilerda. To the south of Ilerda the mountains which adjoin the left bank of the Ebro approach pretty close to the town; to the northward there stretches on both sides of the Sicoris a level country which is commanded by the hill on which the town is built. For an army, which had to submit to a siege, it was an excellent position; but the defence of Spain, after the occupation of the line of the Pyrenees had been neglected, could only be undertaken in earnest behind the Ebro, and, as no secure communication was established between Ilerda and the Ebro, and no bridge existed over the latter stream, the retreat from the temporary to the true defensive position was not sufficiently secured. The Caesarians established themselves above Ilerda, in the delta which the river Sicoris forms with the Cinga (Cinca), which unites with it below Ilerda; but the attack only began in earnest after Caesar had arrived in the camp (23 June). Under the walls of the town the struggle was maintained with equal exasperation and equal valour on both sides, and with frequent alternations of success; but the Caesarians did not attain their object-- which was, to establish themselves between the Pompeian camp and the town and thereby to possess themselves of the stone bridge-- and they consequently remained dependent for their communication with Gaul solely on two bridges which they had hastily constructed over the Sicoris, and that indeed, as the river at Ilerda itself was too considerable to be bridged over, about eighteen or twenty miles farther up.
Caesar Cut Off
When the floods came on with the melting of the snow, these temporary bridges were swept away; and, as they had no vessels for the passage of the highly swollen rivers and under such circumstance the restoration of the bridges could not for the present be thought of, the Caesarian army was confined to the narrow space between the Cinca and the Sicoris, while the left bank of the Sicoris and with it the road, by which the army communicated with Gaul and Italy, were exposed almost undefended to the Pompeians, who passed the river partly by the town-bridge, partly by swimming after the Lusitanian fashion on skins. It was the season shortly before harvest; the old produce was almost used up, the new was not yet gathered, and the narrow stripe of land between the two streams was soon exhausted. In the camp actual famine prevailed--the -modius- of wheat cost 50 -denarii- (1 pound 16 shillings)--and dangerous diseases broke out; whereas on the left bank there were accumulated provisions and varied supplies, as well as troops of all sorts--reinforcements from Gaul of cavalry and archers, officers and soldiers from furlough, foraging parties returning--in all a mass of 6000 men, whom the Pompeians attacked with superior force and drove with great loss to the mountains, while the Caesarians on the right bank were obliged to remain passive spectators of the unequal conflict. The communications of the army were in the hands of the Pompeians; in Italy the accounts from Spain suddenly ceased, and the suspicious rumours, which began to circulate there, were not so very remote from the truth. Had the Pompeians followed up their advantage with some energy, they could not have failed either to reduce under their power or at least to drive back towards Gaul the mass scarcely capable of resistance which was crowded together on the left bank of the Sicoris, and to occupy this bank so completely that not a man could cross the river without their knowledge. But both points were neglected; those bands were doubtless pushed aside with loss but neither destroyed nor completely beaten back, and the prevention of the crossing of the river was left substantially to the river itself,
Caesar Re-establishes The Communications
Thereupon Caesar formed his plan. He ordered portable boats of a light wooden frame and osier work lined with leather, after the model of those used in the Channel among the Britons and subsequently by the Saxons, to be prepared in the camp and transported in waggons to the point where the bridges had stood. On these frail barks the other bank was reached and, as it was found unoccupied, the bridge was re-established without much difficulty; the road in connection with it was thereupon quickly cleared, and the eagerly-expected supplies were conveyed to the camp. Caesar's happy idea thus rescued the army from the immense peril in which it was placed. Then the cavalry of Caesar which in efficiency far surpassed that of the enemy began at once to scour the country on the left bank of the Sicoris; the most considerable Spanish communities between the Pyrenees and the Ebro--Osca, Tarraco, Dertosa, and others--nay, even several to the south of the Ebro, passed over to Caesar's side.
Retreat Of The Pompeians From Ilerda
The supplies of the Pompeians were now rendered scarce through the foraging parties of Caesar and the defection of the neighbouring communities; they resolved at length to retire behind the line of the Ebro, and set themselves in all haste to form a bridge of boats over the Ebro below the mouth of the Sicoris. Caesar sought to cut off the retreat of his opponents over the Ebro and to detain them in Ilerda; but so long as the enemy remained in possession of the bridge at Ilerda and he had control of neither ford nor bridge there, he could not distribute his army over both banks of the river and could not invest Ilerda. His soldiers therefore worked day and night to lower the depth of the river by means of canals drawing off the water, so that the infantry could wade through it. But the preparations of the Pompeians to pass the Ebro were sooner finished than the arrangements of the Caesarians for investing Ilerda; when the former after finishing the bridge of boats began their march towards the Ebro along the left bank of the Sicoris, the canals of the Caesarians seemed to the general not yet far enough advanced to make the ford available for the infantry; he ordered only his cavalry to pass the stream and, by clinging to the rear of the enemy, at least to detain and harass them.