Napoleon's invasion of Russia. It led what might be the first great infographic ever though. Charles Minard’s Infographic of Napoleon’s Invasion of Russia from 1869 (Carte figurative des pertes successives en hommes de l’Armée française dans la campagne de Russie en 1812-1813)

Tan colour line from left to right is the trip from France to Moscow, 1mm line weight = 6000 soldiers, black colour line from right to left is the trip back to France. The line slowly thins and diverges like a tree branch until 422k soldiers are whittled down to 10k returning. Not quite the outcome Napoleon had intended.