When the Americans wanted to take action against the Serbs for their policy of genocide against Bosnian Muslims, the Russians opposed it. This forced the USA to use military force against the Serbs without obtaining approval from the UN Security Council.
Russia would have used their veto in the UN Security Council to block the USA. But giving Milosovic a free hand to continue killing Bosnians was not something Clinton could stomach.
So why did Russia oppose what most people would call a humanitarian obligation? Their official line was that it was an internal matter, and Serbia had the right to do whatever they saw fit. Another objection the Russians have is that their strategic spheres of influence in Eastern Europe were being attacked by western interference. Having witnessed the break up of the Soviet Union, Putin will not tolerate Balkan states aligning themselves with the west, whether that be the European Union or NATO.