up but attacks on the bridges at Maastricht had been costly failures, the day bombers being reduced to aircraft by 12 May. At on 13 May, the began bombing the French defences around Sedan and continued for eight hours with about in the biggest air attack in history. Little material damage was done to the Second Army but morale collapsed. In the French 55e Division at Sedan, some troops began to straggle to the rear and in the evening panic spread through the division. German troops attacked across the river at and had gained three footholds on the west bank by nightfall. The French and the RAF managed to fly and sorties on the Sedan bridges on 14 May but only in formations of The RAF lost aircraft and the French were reduced to sending obsolete bombers to attack in the afternoon, also with many losses. On 16 May, the 1st Army Group was ordered to retreat from the Dyle Line, to avoid being trapped by the German breakthrough against the Second and Ninth armies but on 20 May, the Germans reached Abbeville on the Channel coast, cutting off the northern armies. BEF retreat The plan for the BEF withdrawal was that under cover of darkness, units would thin-out their front and make a phased and orderly withdrawal before the Germans realised what was happening. The objective for the night of 16/17 May was the Charleroi to Willebroek Canal (the Line of the Senne), the following night to the River Dendre from Maubeuge to Termonde and the Escaut to Antwerp (the Dendre Line), and finally on 18/19 May, to the Escaut from Oudenarde to Maulde on the French border (the Escaut Line). The order to withdraw was greeted with astonishment and frustration by the British troops who felt that they had held their own, but they were unaware of the deteriorating situation elsewhere. The withdrawal went mainly according to plan but required hard fighting from the corps rearguards. A communication breakdown caused a loss of co-ordination with the Belgian Army to the north-west of II Corps and a dangerous gap opened up between the two; fortunately it was covered by British light armour before the Germans could discover and exploit it. Loss of the construction divisions The three Territorial divisions, which had arrived in April equipped only with small arms, intended for construction and labouring tasks, were distributed across the