Complex sentence: The Garde Mobile could only be relied upon when defending a fortified position and were almost useless for an attack launched in the style of the Napoleonic Wars, however almost the entire French regular army was already lost at the battles of Sedan and Metz.
Split into simpler sentences: The Garde Mobile could only be relied upon when defending a fortified position and were almost useless for an attack launched in the style of the Napoleonic Wars. However, almost the entire French regular army was already lost at the battles of Sedan and Metz.