The war in Yugoslavia (Serbia) was another energy war. The population of Kosovo, in the southern part of Serbia, is 90% Muslims of Albanian origin as you can see at the following Wikipedia link. These people wanted to be independent from Serbia after the end of the communist period
These separatists were embraced by the Americans and the Europeans. Before the creation of the Former Yugoslav Republic of Macedonia (brown region) and the creation of Kosovo (green region), were both former parts of Yugoslavia. Yugoslavia together with Greece, both close Russian allies, could block oil or natural gas pipelines that could transfer Caspian oil or natural gas from the Black Sea to the Adriatic Sea, since these American and European pipelines would antagonize the Russian oil and natural gas industry.
As you can see at the following link, the AMBP pipeline, is an oil pipeline transferring Caspian oil to the Adriatic Sea, following the route Ajerbaijan-Turkey-Bulgaria-FYROM-Albania-Adriatic Sea-Europe (black line on the map), which has been already agreed, but which has not been developed yet, as you can read at the following Wikipedia link.
Hence, the Kosovars of Albanian origin started protesting requiring their independence, the Serbs used violence against them, and on February of 1998, a NATO operation under the code name Operation Allied Force attacked Serbia. The attacks were carried out by 13 countries, namely U.S.A., Canada, Great Britain, France, Germany, Belgium, Netherlands, Spain, Portugal, Italy, Norway, Denmark and Turkey.
These countries attacked Serbia and forced Serbia to accept an autonomous Kosovo. A large NATO base was established in Kosovo, namely the Kosovo Force, which would see that Albanian Kosovars were not threatened by the Serbs, and that any future pipelines crossing the region would be secure too. You can read about Kosovo Force in the following Wikipedia link.
The following link is a good chronological summary of the NATO attack in Serbia.