Extensive war preparations like those Felgengauer describes do not prove that a definite decision has been taken in favor of war, only that the leadership wants to have the option of war available. The US has made extensive preparations for war with Iran ("preparing the battlefield"), but as yet there is no definite decision to go to war.

To be more precise, extensive preparations usually make available a range of more or less far-reaching military options. The option finally chosen can then be made dependent on circumstances as they evolve. Thus, for the Russian operation in Georgia a minimum option might have been confined to introducing forces into Abkhazia and South Ossetia to back up recognition of their "independence." A maximum option might have entailed occupying Tbilisi and enforcing regime change -- though not, I suspect, advancing south of Tbilisi because this would have threatened core Western interests (i.e., the oil and gas pipelines). What happened in the event was an option between these two.

I think that the two key aspects of the evolving situation that may have influenced the choice of option were:

(1) the Georgian reaction to the creeping annexation of South Ossetia in its various aspects (making more Ossets Russian citizens, expulsion of inhabitants of Georgian villages in SO, etc.)

(2) the reaction of the West.

Given a Western reaction restricted to rhetoric, I think that if there had been no significant Georgian reaction the operation would have been limited to the minimum option, as defined above. This would not have attracted so much world attention and would almost certainly have sufficed to block Georgia's accession to NATO, which is after all the main Russian goal. The political costs of going further under those circumstances would have exceeded the military benefits.

However, the Georgian attack on Tskhinval sharply changed this calculus in Russia's favor. It meant that even if a more ambitious option was selected world opinion would be confused and divided. Instead of Russia finding itself isolated internationally for clear-cut aggression, many people throughout the world (most importantly in Europe) sympathized with the Osset victims and adopted a position equidistant between the sides.