The lines of France and Germany cross at around 1870 so their populations were equal that's true. But you also have to consider the age distribution. The line for Germany has a steeper slope throughout the decades before and street 1870 which means they were growing at a significantly faster rate and for a while. So their age distribution must have had more younger people than France at the same time. Soldiers are mostly recruited from the age cohort 18-30 and 1870 Germany must already have had more people in that age cohort than France despite total population numbers being around equal.The interesting thing about this graph is that in 1871 the Germans had no demographic advantage over the French. Sure the French were no longer the most obviously populous nation in Europe any more, but they were not significantly outweighed by the Germans, as they would be by 1914. The cause of French loss of power is something more than just demographic.
That being said - ofc France's defeat in 1870 wasn't primarily due to demographics The Prussian army just did the whole "hit them first and hit them hard" perfectly while the French army dropped the ball. Could have been a reverse result had the quality of preparation been reversed.
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