The key elements are the political pressure and nuclear weapons. Everything else is seconday. Keep in mind that Russia's military doctrine emphasizes nuclear weapon use, even preventively.
Russia has to fear a limited war in which it's pitted against a proxy of European and American forces with NATO providing air support to their allies. For example if Russia were to have to occupy Georgia (please don't tell me it's not going to happen, I know that, I'm just giving an example of a nightmare scenario for the Russian Army).
EDIT: Darth I would appreciate a reply to my earlier statement in regards to AD network capabilities. Your opinions have been very interesting so far, and I would like to continue the discussion.
I think the Russian AD network is probably very dangerous. If I had to guess they have multiple EW radars covering Europe, they could put AEW aircraft up at will. They have comprehensive coverage and protection against cruise missiles in the form of the Mig-31 and other hard kill systems. It's impractical to cover every sq km of frontage even with a gazillion air defense systems so they with use intel and poor OPSEC to determine likely air avenues of approach. Thats where you will encounter the most serious concentrations of air defenses and fighters. The cities and fixed sites will have the most coverage of systems like the S-300 ect.
The ground combat forces and mobile assets will have organic mobile battlefield SAMs and AAA. In the vicinity of high value targets is where you will find the short ranged missiles and point air defense systems. They will use fighters(Su-27/Mig-29) to cover gaps and dead space. All of this will be redundantly networked to some form of C3. I don't know the exact configuration but I would guess its a combination of land lines and RF connections.
All of that will be supported by HUMINT/ELINT/COMINT/IMINT all the way back to the source. For example if a conspicuous number of tankers start collecting at bases near their territory it could be hard to conceal. They will dig deeper and that could be an issue in FSU states where FSB and other intel services likely still have contacts. They will adjust off of that information.
Other than the technological and numerical differences when compared to previous IADS western aircraft have dealt with the Russians will also have considerable offensive capabilities that can reach all the way to Europes west cost if necessary. It will be a lot more effective than Iraqs SCUDs and insurgents.
Challenging the Russian IADS and penetrating into the interior in any kind of sustained campaign would be a very serious undertaking. My question to advocates of this is where would Europe stage for something like this? Poland, Georgia, Estonia, Lavtia, Turkey?
-DA