1st Army (Russian Empire)
It was commanded by General Paul Rennenkampf when Russia invaded East Prussia at the outbreak of war in 1914. They were accompanied by the Second Army commanded by General Alexander Samsonov. The plan was to trap the German army in a Pincer movement between the two armies. On 17 August the First Army engaged the Germans at Stallupönen. They fought the Germans again on 20 August at Gumbinnen and defeated them. Rennenkampf thought the Germans could be finished off by Samsonov's Second Army and did not move quickly enough to help them. In late August 1914 the German and Russian second army met at Tannenberg. The Second Army was encircled and suffered complete destruction.