Hi, you may be interested in this discussion of the Mongol invasion, in contrast with Napoleon and Hitler:

• How come the Mongols were able to invade Russia in the winter, when Napoleon and Hitler failed?

Well it is hard to call the Central Powers fighting in WW1 anything but a success in the end, with major individual victories at places like the Marsurian Lakes.

While Russia got in some good licks, such as the Brusilov Offensive, and essentially gutted the Austrian army in 1916, and forcing an ever larger German commitment. But in the end the battlefield defeats and constant pressure added up to contribute to the domestic problems. Thus the Russian Revolution, and while the Bolsheviks tried to carry on the war, in late 1917 they too saw the writing on the wall and sued for peace.

Then when negotiations dragged the Germans resumed the offensive and gobbled up more territory. By the end Germany had basically all the Baltic s, Poland, Belarus, Ukraine, and then some, key parts of the Tsarist Empire. And while the situation would not last past November 1918, and even still as it happened were a major headache and manpower drain, the fact is Germany came out ahead because the Russia she faced in 1914 could not survive the stress of an invasion and World War on her society and government.