Tension in the Caucasus has been building up for a while, and one must have been blind to say the war is a surprise. At least since 2006 there have been skirmishes in South Ossetia and diplomatic strain, to put it euphemistically, between Russia and Georgia.
You ask why US didn't alert the poor helpess georgians of the russian threat, but wait! Georgia started the conflict by assaulting South Ossetia. Analysts have suggested that they did so expecting the US to back them up. As anyone knows, current georgian president Mikheil Saakashvili is closely aligned with the U.S.
On the surface, it may look like a simple question of secession, which does exist and is quite old: South Ossetia is a province of Georgia, with many ethnic russians, that has declared its indepenence in 1992, though it was not internationally recognized. Another dissident region is Abkhazia, in a similar political status, but effectively under abkhaz rule since georgian defeat in 1992-94. It has also been said that the recognition of independence of Kosovo by western countries, which certainly upset Russia, historical ally of Serbia, may be used as justification for russians to support the ossetian and abkhaz separatism.
But the underlying theme here is obviously Russia's struggle not only to maintain its area of geopolitical influence, threatened by moves toward west in Ukraine and Georgia, but to gain absolute control over the transportation of oil and gas. That's what explains russian immediate and asymmetric response, which went way beyond "protecting ossetians from Georgia". That's also why Sarkozy, representing the european countries which depend heavily on the supply of oil and gas from that region, hurried in to establish a cease-fire.
But let us not forget that it is us, western countries, that wish to reinforce georgian territorial integrity, denying the dissidents their independence for the sake of of a strong ally in a strategical region. There are no good guys or bad guys. I am not defending russian imperialism as I wouldn't defend british or american or corporate imperialism (the three REAL ones in rough chronological order).
Maybe the US doesn't care enough about their georgian friends (they certainly don't care about the ossetians...) to get involved. That's ungrateful if you think Georgia sent troops to support the invasion of Iraq. Maybe they're too busy with two pointless wars (three with the war on drugs), or bringing back to life the Fourth Fleet to oversee potential deep-sea oil reservoirs being discovered in the Atlantic. Or maybe they ARE indeed preparing for a conflict against Iran, both directly and/or through the mediation of Israel.