Russia has slowly been flexing it's muscles for a while now, though as far as I know didn't Georgia kind of start this one, or at least some Georgian separtists? Correct me if I'm wrong there.
It's a complicated situation. South Ossetia is a semi-automonous region within Georgia's borders. Most of the populace are ethnic Russian and would like to join up with Russia. Russia had gone so far as to issue Ossetians Russian passports.
The Georgians wanted to retake South Ossetia and assert their national sovereignty over the region. They signed a cease-fire agreement with the Ossetians, and broke it with a sneak attack. So, Georgia is at least partly to blame.
But South Ossetia is inside the internationally recognized borders of Georgia. So, any military action inside their borders by Russia is an invasion against a US ally, regardless of whether the Georgians had clean hands or not. And few outside of Russia believe their intentions towards South Ossetia are humanitarian.
The situation has calmed down, temporarily at least. But if Russia is ultimately allowed to annex these two territories, fears that they are seeking to rebuild the old Soviet empire will rise.