Complex sentence: His divisions were briefly attached to the retreating First and Second Armies in Lorraine before being quickly redeployed by rail on 26 August further northwest to form the new Sixth Army north of the Western Front.
Split into simpler sentences: His divisions were briefly attached to the retreating First and Second Armies in Lorraine. They were quickly redeployed by rail on 26 August further northwest to form the new Sixth Army north of the Western Front.