Throughout the early summer of 1711 Marlborough's army, having taken the important fortress of
Douai the previous year, manoeuvred indecisively in northern France, blocked by the French Lines of Ne Plus Ultra – a massive series of fieldworks stretching from the
Channel coast to the
Ardennes at
Namur. The allied army had been weakened by the withdrawal of
Prince Eugene's army to cover the upper
Rhine, as the deposed
Elector of Bavaria attempted to take advantage of the disruption caused by the death of the Emperor
Joseph. On 6 July, Marlborough captured the small fortress of Arleux, just to the north of the Lines, west of Bouchain, both to deny its use to the French as a sally-port, and to secure the water supply to Douai, which could be cut off by damming the canal that supplied the town. The Duke was then wrong-footed by Villars as the French army crossed the Lines on 22/23 July and retook Arleux, with the allied army too far to the west to intervene in time, and the defences were levelled before the French retreated back across the Lines. Marlborough, initially furious, soon retook the initiative by marching his army as if to assault the Lines near
Arras, and carrying out a detailed personal reconnaissance there on 4 August in full view of Villars' covering army. That night the army struck camp, leaving their campfires burning to deceive the French, and marched eastwards to Arleux. At midnight a force from Douai under
Cadogan crossed the unguarded French lines, and by 8 am the advance guard of the main army was also crossing over. Villars, arriving on the scene with a few hundred cavalry, realised he had been outmanoeuvred, and though he attempted to offer battle in front of Bourlon Wood, Marlborough declined to attack, the Marshal's position being even stronger than the one in which he had given Marlborough's army such a mauling two years earlier at
Malplaquet. He thus drew off and attempted to hinder Marlborough's siege of Bouchain which followed. ==Siege==