21日早晨,法军总指挥约瑟夫·霞飞向朗雷扎及英国远征军告知德军正在向西进军。根据第十七号计画,法国第三及第四军团应分别向南移师阿尔隆及讷沙托,并在卢森堡省寻求攻击任何敌军。法国第五军团被下令防守掩护默兹河到那慕尔的路上,而英军则透过向蒙斯东北的苏瓦尼进军来配合此一行动。有鉴于此,兰雷扎克将麾下的第五军团部署在桑布尔河,并在当天稍晚的12点30分向霞飞回报他的行动。当时他并不知道,德军已与他的先锋部队在那慕尔和沙勒罗瓦间发生冲突。下午两点,朗雷扎才在那慕尔指挥官奥古斯坦·米歇尔(英语:Augustin Michel)将军的报告中意识到了这一点。下午四点,总部告知兰雷扎克德军仍在向西进军,随后命令他的空军侦察敌军行动,并告知其麾下部队“做好准备借由渡过桑布尔河向那慕尔及尼韦尔发起进攻(be ready to launch an attack [...] by crossing the Sambre, towards Namur and Nivelles.)”[4]。朗勒扎克在晚上七点向霞飞回报第十兵团前线仅有小型行动[5],于是后者在晚上八点指示朗雷扎有完全的自由裁量权来决定何时发起进攻[6]。
德国第三军团横渡默兹河并向由第一兵团(英语:1st Army Corps (France))驻防的法军右翼进攻。德军这次的进攻威胁到第五军团并有可能切断其撤退路线,但第一兵团成功在反击中阻挡了德军的推进。随著盟军从那慕尔撤离以及第四军团从阿登地区撤退,朗雷扎也下令第五军团撤退,以避免他们被包围并与法军其馀部分失去联系。这场战役最终以德军胜利告终。
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