1914
Battle of Tannenberg
The setting
Near Allenstein (Olsztyn), East Prussia, German Empire (now Poland)
Rolling East Prussian country broken by the Masurian Lakes and forests, with a dense German rail network - features that channeled movement and let the defender shuttle forces rapidly between fronts.
Reveal the outcome
Recorded sideRussia (2nd Army under Samsonov)
Recorded resultCatastrophic defeat; army largely destroyed, Samsonov died (reportedly by suicide)
The German Eighth Army encircled and destroyed most of the Russian Second Army; tens of thousands were killed or captured and the army's commander Samsonov died (reportedly by suicide).
How the atlas explains it
The German command exploited intercepted, largely unencrypted Russian radio traffic to read enemy movements, used rail mobility and lake terrain to concentrate against the isolated 2nd Army, and executed a double envelopment that trapped and annihilated it before the other Russian army could intervene.
What followed
A crushing early-war German victory that wrecked the Russian invasion of East Prussia, made the reputations of Hindenburg and Ludendorff, and became a defining symbol of envelopment battle - though it did not change the war's overall course.