On 5 September the Sixth Army advanced eastwards from Paris and met the German IV Reserve Corps, which had moved into the area that morning and stopped the French short of high ground north of Meaux. Overnight the IV Reserve Corps withdrew to a better position east and French air reconnaissance observed German forces moving north to face the Sixth Army. General Alexander von Kluck the 1st Army commander, ordered the II Corps to move back to the north bank of the Marne, which began a redeployment of all four 1st Army corps to the north bank by 8 September. The swift move to the north bank prevented the Sixth Army from crossing the Ourcq but created a gap between the 1st and 2nd Armies. The BEF advanced from 6 to 8 September, crossed the Petit Morin, captured bridges over the Marne and established a bridgehead deep. The Fifth Army also advanced into the gap and by 8 September crossed the Petit Morin, which forced Bülow to withdraw the right flank of the 2nd Army. Next day the Fifth Army re-crossed the Marne and the German 1st and 2nd armies began to retire as the French Ninth, Fourth and Third armies fought defensive battles with the 3rd Army which was forced to retreat with the 1st and 2nd armies on 9 September.