Following the outbreak of war it was mobilised in early August and placed under the command of General Alexander Samsonov. Under pressure from their ally France, who was facing a powerful German invasion force in the west, it was decided that the 2nd Army would join the 1st Army as part of General Yakov Zhilinskiy's Northwestern Front where it would participate in the upcoming invasion of East Prussia. The combination of the 1st and 2nd Armies had a fatal flaw; Samsonov and the commander of the 1st Army, Paul von Rennenkampf, had had an antagonistic and hostile relationship since 1905.