The Napoleonic Wars were not one long Allied thrashing of France. French forces defeated multiple coalitions for much of the period, and the final reversal came when their opponents combined and cooperated more effectively. The wars themselves are therefore an insufficient cause of France's later military difficulties. For the conditions visible by the First World War, the political settlement and organization created after Napoleon III offer a more useful explanation, although this does not amount to praise for Napoleon III.
Notable Point · 3:52:54-3:53:37
Napoleonic defeat does not explain France's later military problems by itself
France dominated much of the Napoleonic Wars, so later weaknesses require attention to the post-Napoleon III settlement and force organization.