The Battle of Sedan on September 1, during the Franco-Prussian War, brought an end to the Second Empire and the reign of Napoleon III and the war itself was lost fairly soon afterward.
But Sedan wasn’t just bad for France. It helped convince European governments and much of the public that the next major war would also be quick, marked by decisive battles of manoeuvre.
They might have paid more attention to the long, bloody American Civil War and even the carnage of the Russo-Japanese War if it weren’t for the sudden collapse of the French war effort at Sedan. And if they had, just maybe they’d have worked harder to prevent World War I.