Russia has intervened directly in favor of secessionist rebels in the Georgian region of South Ossetia (center of map). The Russians have always helped and supported the secessionists, in both South Ossetia and in Abkhazia (top left map). In 1995, both regions (with Moscow's connivance) achieved de facto if not de jure independence from Georgia, and Russian "peacekeepers" have been in the area since at least that time.
Georgia has never given up trying to reintegrate both regions, and Georgian military forces earlier this week occupied the secessionist capital at Tskhinvali (see map), and clamed the city had been "freed." The Georgian President, Mr. Mikheil Saakashvili, says that Russian Ground Forces then crossed the border. The Georgians claim the force includes armor (the Reuters story says "150 tanks" plus APC's and other vehicles), and that Georigan military airfields around their capital Tbilisi (center right on the map) have been bombed. Russian news agencies claim that a Russian armored column has moved over the border. The Georgians claim to have shot down two Russian aircraft.
Apparently shelling from heavy guns is on-going near Tskhinvali. Owners of the guns unknown.
Unlike the United States, the Russians are very careful about giving out too much order-of-battle information, so any information has to be taken with a box or two of salt. Still, here is what is publicly available about Russian Ground Forces in the Caucasus/Transcaucasus region.
Georgia borders the Russian North Caucasus Military District. This military district probably has most of the combat-capable ground units in the Russian Ground Forces, simply because it is tasked with ongoing Russian operations in Russia's own breakaway Chechnya region. Supposedly, it has a number of mountain-warfare capable units.
Just across the Georgia border, in North Ossetia, is the Russian Ground Forces 58th Army (wich is subordinate to the North Caucasus MD, and is its principal Ground Forces unit. The principal maneuver unit in this army is the 19th Motor Rifle Division, which has three Motor-Rifle [infantry] regiments, a separate tank regiment [90-100 tanks, all up] and a self-propelled artillery regiment (big guns on tracked vehicles).
Additionally, 58th Army has at least three subordinate separate motor rifle brigades or regiments (maneuver elements about a third a size of the 19th MRD at full strength). However, the Russian Ground forces are not the Soviet Ground Forces, and they do not possess the wealth of personal and material of similar units in Soviet times. Most units are cadres -- that is, greatly below their full notional strength, and are filled-out with additional troops and equipment when needed.
Although there are at least two other MRD's in the North Caucasus MD, in addition to substantial Interior Ministry paramilitary units, much of what's there (including 58th Army) have commitments in Chechnya. In all probability, if a major intervention in Georgia is seriously contemplated, and continues for any length of time, other forces will be required.
Georgia's land forces are about 20,000 strong, comprising four infantry brigades and supporting units. Two additional brigades are supposed to be raised by 2010. The Georgia Land Forces have enjoyed good relations with the US military (including joint exercises and training) and sent forces to participate in Operation Iraqi Freedom. (Wretchard at Belmont Club has a good discussion of US/Georgia military relations, and this new crisis, here). Georgia can probably defend itself against anything short of a full-scale invasion, simply because of geography, but if the Russians are determined, Georgia is in trouble.
Georgia is not part of the North Atlantic Treaty Organization, but has a "partnership for peace" with NATO, whatever that might mean.
What appears to have happened today is that Georgia decided that it would be able to bring separatist South Ossetia back under its control without Russian objection. Georgia's calculation appears to have been erroneous. Probably, the Russians implemented standing plans for their own military response in the event Georgia tried this.
The decision to execute the plan would go all the way to Moscow, to Russian President Dmitry Medvedev and to Prime Minister Vladimir Putin. Medvedev and Putin appear to think they have a green light to move troops openly into a sovereign state that has relations with NATO; or that NATO (meaning the US) is too busy elsewhere to really object, much. Why the Russians might think that should cause some other people to think a little bit. . .