President Dmitri Medvedev of Russia said Thursday that Russia would act as an international guarantor of South Ossetia and Abkhazia, the two pro-Russian enclaves at the center of the crisis that have long desired separation from Georgia. As Secretary of State Condoleezza Rice headed to the region for discussions on the crisis and to show support for President Mikheil Saakashvili of Georgia, the Russian position seemed to be a direct challenge to President George W. Bush, who had said the day before that he "insists that the sovereign and territorial integrity of Georgia be respected." In Georgia, meanwhile, Russian forces...