Background:
[1991-1992: Civil War in South Ossetia, and pretty much Georgia as a whole]
November 1991 - Russian Parliament threatens Georgia with economic sanctions if failed to take steps to end the conflict and restore South Ossetia’s autonomy.
[...]
24 June 1992 - Shevardnadze and Yeltsin meets to discuss the question of South Ossetia and agree in principle on a cease-fire and the establishment of a Joint Control Commission.
[...]
14 July 1992 - Russian peace-keeping operation starts (three-sided peace-keeping forces - Russians, Georgians and South Ossetians) upon agreement between Russian government, the Georgian State Council and of the Supreme Soviet of South Ossetia.
[...]
8 October 1993 - Georgia joins the Commonwealth of Independent States (SNG) and makes agreement on Russian military bases in Georgia.
[...]
15 August 1994 - CSCE (OSCE) proposal for a framework for a constitutional model as a political solution for the Georgian - South Ossetian conflict. The proposal is rejected by both parties.
[...]
May 1996 - Shevardnadze and the South Ossetian leader Ludwig Chibirov sign a ‘Memorandum on measures for providing security and joint confidence’ in which the two sides renounce the use of force.
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Hence why:
a) no Georgian forces in South Ossetia (other than some police working jointly with South Ossetian units)
b) some 500 Russian peacekeepers in South Ossetia
c) independant official South-Ossetian militia
d) defined (and checked) border between Georgia and semi-autonomous region of South Ossetia
Georgia sent units across this border, violating the 1996 memorandum, attempting to occupy South Ossetia militarily, and in the process attacking and isolating Russian peacekeeping units; Russia officially uses these attacks to militarily "re-open" LOC and LOS to its peacekeeping forces in South Ossetia, and removes all "threats" to this peacekeeping force.