To fulfill their obligation to the Allies, Russia attempted an invasion of German territory as early as they could, sending the 1st Army under Rennenkampf and the 2nd Army under Samsonov against East Prussia in a huge 'pincer' movement, 1st Army approaching from the east, north of the Masurian Lakes area, and the 2nd attacking north, from southwst of the lakes.
The only hope for the German 8th Army (Prittwitz) covering East Prussia was to attack and defeat each of these armies in turn, not wait till they could concentrate or envelope.
Initial confrontations at Stalluponen and Gumbinnen did not bode well for the Germans and Prittwitz ordered a retreat. This was never fully carried out, as he was relieved, and Hinderburg & Ludendorf took over. They recognised the danger and reversed the retreat. While Rennenkampf's 1st army paused waiting for the Russian supply mechanism to catch up, the bulk of 8th Army with reinforcements from the Western Front rushed south and caught the 2nd Army at Tannenberg, completely enveloping them, capturing most and sending the rest running back to Warsaw. Ludendorf then turned his attention to the Russian 1st Army and while the first battle of the Masurian Lakes drove the Russians back, it was an orderly retreat along their lines of supply enabling them to launch a counter-offensive pushing the Germans back to the frontiers.
A campaign that saw heavy casualties amongst the poorly prepared, supplied and equipped Russians and made the reputations of Hindenburg and Ludendorf but succeeded in diverting German troops from the campaign in France, thus putting the final nail in the coffin of the Schlieffen Plan.

• Stallupönen 17 August 1914
• Gumbinnen 20 August 1914