Russian invasion of East Prussia

According to the terms of their alliance with the rest of Entente, Russian Empire invades Eastern Prussia in force.

Russian Empire mobilized more slowly than the better-organized countries of Central Powers and rest of Entente. Because of that (and of regional opposition to the draft, a part of generally cool attitude towards the war within the lower classes of the Empire), they were a bit late to the festivities.   Russian Empire planned to attack Central Powers from the east, drawing its attention from the Western Front and overwhelming the numerically inferior forces of the German Empire left to defend its eastern border. First move (besides pushing back Austro-Hungarian Empire) was to take down the East Prussia. This would defend the northern flank of Central Poland, allowing the Russian Empire to prepare its next offensive from the Warsaw region towards Berlin.   Two armies entered East Prussia, under commands of generals Alexander Samsonov and Paul von Rennenkampf. While initially successful - due to massive numerical superiority - the situation changed a lot after Paul von Hindenburg and Erich von Ludendorff were sent to replace the current commander of eastern German forces.   The invasion was halted. During the Battle for Tannenberg Alexander Samsonov's Second Army suffered heavy casualties and was almost broken. It wasn't until the much-needed relief - an attack from the von Rennenkampf's First Army - came that the Second Army's encirclement was broken.   Ultimately, however, the campaign ended in the decisive victory of the German Empire. Samsonov's army stopped being a useful fighting force at least for a while. Rennenkampf's army was ultimately defeated in the First Battle of the Masurian Lakes, which led to the Russian Empire's forces being pushed out of East Prussia.   It can be speculated as to what would happen if extremely unprepared and disorganized Russian Army was defeated more decisively. Rennenkampf's German-sounding name could lead to sudden surge of anti-German sentiments among the masses. What's more, this would mean grievous casualties to best and most loyal forces of the Empire - those loyal directly to the Tsar, even more than the Empire or Russia. If Russia was forced to draft officers from politically unsure groups earlier on, then both Revolutions would have happened earlier and the Russian Army would be broken much more and much more early.