>Belgrade’s position to not recognize the breakaway Donbass republics comes in view of Serbia’s own province of Kosovo, occupied during the US-led NATO bombing campaign of Yugoslavia in 1999, declaring independence in 2008 with US support. While over 100 countries, mainly US allies, have recognized Kosovo as independent, both Belgrade and Moscow have refused to do so.
Some history stuff…
Muh Wiki
NATO bombing of Yugoslavia
The North Atlantic Treaty Organisation (NATO) carried out an aerial bombing campaign against the Federal Republic of Yugoslavia during the Kosovo War. The air strikes lasted from 24 March 1999 to 10 June 1999. The bombings continued until an agreement was reached that led to the withdrawal of Yugoslav armed forces from Kosovo, and the establishment of the United Nations Interim Administration Mission in Kosovo, a UN peacekeeping mission in Kosovo. The official NATO operation code name was Operation Allied Force whereas the United States called it Operation Noble Anvil;[26] in Yugoslavia the operation was incorrectly called Merciful Angel (Serbian: Милосрдни анђео / Milosrdni anđeo) as a result of a misunderstanding or mistranslation.[27]
https://en.wikipedia.org/wiki/NATO_bombing_of_Yugoslavia