Late August 1914, forest and lakes in East Prussia. Two Russian armies have marched in without talking to each other. Hindenburg and Ludendorff, names the world has not yet learned, close the trap on one of them. Its commander, Samsonov, walks into the woods and shoots himself.
Why then
Germany's plan assumed Russia would take six weeks to mobilise. Russia is already in East Prussia, which is why Moltke pulls two corps from the drive on Paris. What decides the battle is small and technical: Russian radio orders sent uncoded, and a staff that cannot find its other wing.
What they wanted
The Tsar needs a victory that saves France. Prittwitz, the German commander on the spot, proposes a retreat and is sacked for it. Ludendorff plans an encirclement on the classical model, one that a staff officer, Max Hoffmann, has already sketched.
What changed
One Russian army is destroyed, with over 90,000 taken prisoner, while Belgium is still burning. The east will swallow millions more for three years, then end in a separate peace that frees German divisions for a last throw in the west in 1918.
Still argued
The victors made it a legend and named it after a medieval defeat they wanted to avenge. Historians since have given less weight to German genius and more to the Russian mistakes.