So, throughout history, Russia has been invaded many times. From the Mongols, to the Swedes, to the French, and lastly, by the Germans. However, every time they have failed. The only one who was able to control (in a fairly loose term of the word) Russia for any extended period of time was the Mongols, or the Golden Horde, if you prefer, and that was only due to their high mobility and letting the Russians rule themselves, for the most part, and even then they were driven out after a hundred years or so. However, Napoleon's invasion set precedent for the next invasions, as it was there that the Russians revealed their main tactic: slash and burn, retreat, wait till General Winter, their most deadly ally, sets in, harass, then advance. Even when the Germans invaded, wise of Napoleon's mistake, they could not take Russia.
So, is there any conceivable way to invade, defeat, and occupy Russia? I have been thinking about this for around a year now and I still have not found a solution that does not require the use of nuclear weaponry, which I, and indeed, most of us, would like to avoid using.