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Germany’s Schlieffen Plan left about 10 percent of its army, the Eighth Army, to hold East Prussia, but Russia had promised France 800,000 men in the field by day 15 of mobilization, a task its single-track railways would normally need some 60 days to support. Russian trains stopped at the border because of a different rail gauge, supplies moved by horse cart over sandy tracks, and each cavalry horse consumed roughly ten times a soldier’s needs. With too little telegraph wire, the armies of Paul von Rennenkampf and Alexander Samsonov under front commander Yakov Zhilinsky relied on radio, and Samsonov, who had no copy of the new codebook, kept moving his starving men and sent his orders unencrypted.

After the battle at Gumbinnen on August 20, Maximilian von Prittwitz proposed retreating to the Vistula and was replaced by Paul von Hindenburg and Erich Ludendorff. Staff officer Max Hoffmann assured them Rennenkampf would not help Samsonov, so they left a thin screen in the east and sent their forces south by rail. Zhilinsky ordered the Russian 6th Corps away from the main army, and between August 26 and 30 the Germans closed a double envelopment. Cut off from his radio, Samsonov walked into the woods on the night of August 30 and killed himself.

• Hoffmann had watched Samsonov and Rennenkampf brawl at a railway station during the Russo-Japanese War.
• Hermann von François’s I Corps traveled over 150 kilometers by trains running every 30 minutes, with 25 minutes to unload each one.
• Germany committed about 150,000 men and suffered about 12,000 casualties, while Russian losses exceeded 120,000.
• Sixty trains were needed to carry the captured Russian guns and equipment back to Germany.
• Most fighting was near Allenstein, but Hindenburg named the battle for Tannenberg, about 30 kilometers away, site of the Teutonic Knights’ defeat in 1410.