How? Under Tito in the so-called Non-Aligned Movement Yugoslavia consistently opposed the United States by diplomacy, voice and vote in every international forum. The Serbs sold weapons to Qadaffi and Saddam Hussein in the 1990s knowing they were our avowed enemies and in violation of sanctions. Slobo even awarded Qadaffi with Yugo's highest medal in 1999. They sent Serb technicians and military advisors to Iraq as recently as 2002.
We fought on the same side in WWI and WWII, but its been pretty bleak since then.