21日早晨，法軍總指揮約瑟夫·霞飛向朗雷扎及英國遠征軍告知德軍正在向西進軍。根據第十七號計畫，法國第三及第四軍團應分別向南移師阿爾隆及訥沙托，並在盧森堡省尋求攻擊任何敵軍。法國第五軍團被下令防守掩護默茲河到那慕爾的路上，而英軍則透過向蒙斯東北的蘇瓦尼進軍來配合此一行動。有鑑於此，蘭雷扎克將麾下的第五軍團部署在桑布爾河，並在當天稍晚的12點30分向霞飛回報他的行動。當時他並不知道，德軍已與他的先鋒部隊在那慕爾和沙勒羅瓦間發生衝突。下午兩點，朗雷扎才在那慕爾指揮官奧古斯坦·米歇爾（英语：Augustin Michel）將軍的報告中意識到了這一點。下午四點，總部告知蘭雷扎克德軍仍在向西進軍，隨後命令他的空軍偵察敵軍行動，並告知其麾下部隊「做好準備藉由渡過桑布爾河向那慕爾及尼韋爾發起進攻（be ready to launch an attack [...] by crossing the Sambre, towards Namur and Nivelles.）」[4]。朗勒扎克在晚上七點向霞飛回報第十兵團前線僅有小型行動[5]，於是後者在晚上八點指示朗雷扎有完全的自由裁量權來決定何時發起進攻[6]。
德國第三軍團橫渡默茲河並向由第一兵團（英语：1st Army Corps (France)）駐防的法軍右翼進攻。德軍這次的進攻威脅到第五軍團並有可能切斷其撤退路線，但第一兵團成功在反擊中阻擋了德軍的推進。隨著盟軍從那慕爾撤離以及第四軍團從阿登地區撤退，朗雷扎也下令第五軍團撤退，以避免他們被包圍並與法軍其餘部分失去聯繫。這場戰役最終以德軍勝利告終。
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