God Save the Tsar

In late August 1914, the Imperial Russian Army launched and invasion of east Prussia. This was done by the First and Second Armies, under the commanders Samsonov and Rennenkampf. In OTL, they hated each other and their feud resulted in the deaths of thousands, as well as a costly defeat for Russia against Germay in the early war. However, this timeline's POD explores the idea that instead of Rennenkampf, the First Army was actually commanded by a different commander. They had no problems with working together and defeated the German Eighth Army at Tannenberg, leading to success. This is God Save the Tsar.