|Russian Troops in East Prussia, September 1914
Officially known as Mobilization Schedule XIX, the plan that the Russian Army implemented in August 1914 is commonly called Plan XIX. The original version was drawn up by the general staff in 1910 and it focused on Germany as the primary enemy, anticipating an invasion out of East Prussia against Russian Poland. The 1910 version, however, was opposed by a number of factions, including those who believed (correctly) that—in the case of war—Germany would focus on France first and by others who saw that with Austria-Hungary being the weaker of the prospective opponents, a Russian advance into Galicia had a greater potential payoff in terms of expanding the empire.
The ensuing debate led to a modification in Plan XIX—approved by the tsar in June 1912—giving two mobilization options with a final choice to be made by day nine. Under Option A, the bulk of the forces would be directed against Austria in the southwest, with a much lesser defensive component facing East Prussia. Under Option G, the Army would take a defensive posture against Austria and shift more forces north to face a German onslaught, if that's the way things developed. Option A naturally made the French, who were also correctly assuming they would be the initial targets of the Germans, nervous. They pushed for a more active and earlier advance by the Russians against East Prussia.
In August 1914 Option A was implemented immediately, but with the Russian 1st and 2nd Armies also advancing on the offense against East Prussia and moving more quickly than anyone anticipated. France got the help she needed; the Russians; however, were headed for disaster with the loss of their entire 2nd Army at Tannenberg.