1st Army (Russian Empire)

The Russian First Army was a World War I Russian field army that fought on the Eastern Front for two years. 
August 1914.
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 and suffered complete destruction.
September 1914.
Let by General Paul von Hindenburg the Germans turned north to attack the Russian First army at First Battle of the Masurian Lakes in September 1914. The Russians fought hard but were driven back to the Russian border. Rennenkampf kept his troops in good order as they retreated. They set up defensive lines on the Russian border.
November 1914.
After another defeat near Silesia on November 11, Rennenkampf was dismissed. He was replaced by General Litvinov. The First Army again fought at Thorn and Lódź but there was no clear cut winner.
December 1914.
The First army evacuated Lódź and returned to their own lines for the remainder of the winter. The First Army served on the Northwestern front for the rest of the war.