In OTL, the Battle of Tannenberg, fought between August 26—30 of 1914, was one of the most important engagements on the Eastern Front of World War I. In it, the German Eighth Army, consisting of some 150,000 men, defeated the Russian Second Army, which had 230,000. It was a devastating defeat and a major turning point in the Eastern Front, demoralizing the Russians and raising the Germans' morale. Several reasons are cited as the cause of the defeat: the poor communications between the Second and First Armies, and the rivalry of their two commanders, causing failure of cooperating during the battle.
But what if they did?
This scenario explores the point of divergence of rather than Paul von Rennenkampf, the Russian Army general and an enemy of Alexander Samsonov, commanding the First Army, a different officer instead took his place. The replacement was Mikhail Alekseyev, an experienced infantry general who had no problems cooperating with Samsonov.