The first true-fact to know is that Partisans in yugoslavia exclusively fought for power durgin the war, which tells much about their loyalty to any nation they belonged to.
That's why their ennemy number one were the Chetniks of Draza Mihailovic who rejected capitulation against german invaders and their allies in and out of yugoslavia. German invaders and their allies came at best second in the priority list camp to fight against.
In Serbia, and in extension, among Serbs, Partisans were brought from outside as there used to be very few support to communism among Serbs. They were brought at the express demand of Stalin and Ante Pavelic separately, during the then still-alive Ribbentropp-Molotov agreement between Nazi Germany and USSR establishing mutual peace and non agression.
Partisans were mostly ordered freedom by Hitler from prisons in France (where they've been jailed after the disastrous spanish civil war in 1936, where international brigades fled Spain by the mountain Pyrénées, hoping to be saved by, at that time the sole leftist socialist-communist government in Europe), to be sent in Serbia, where they could give a help to Germans for stopping the serbians fight for freedom.
Due to these profile and assigned goal, partisans without hesitation fought shoulder to shoulder with oustachis west of the Drina, as commanders of partisans units were all croats, all the soldiers were most exclusively Serbs, some Partisans "officers" having their own brother beeing commanders of Oustashis. Betraying Chetniks east of the Drina in their attacks of German occupiers since 1941, then provoking mass execution of Serbs by doing non-sense and unuseful attacks of second line occupiers administration while was used to be put in practice the rule: "1 german wounded = 50 executed Serbian civilian. 1 dead german = 100 executed Serbian civilian". Not to mention of course that those "heroic" partisans actions where exclusively taken in Serbia among Serbian population, and NEVER on croatian, bosnian muslim, albanian, hungarian territories of yugoslavia.
Partisans also signed an agreement with German forces that they would fight shoulder to shoulder with German nazis, against Allied (UK, USA, etc...) forces if any Allied invasion had to happen in Dalmatia, the then yugoslav Adriatic coast (which was in practice mostly handled by Draza Mihailovic Chetniks).
The reason why, first, an uprise happened in Serbia, while elsewhere in Europe it was very quiet is that Churchill, frightened to death for beeing invaded by Hitler during the Battle for England, tried to make reduce the Hitler's pressure on the Britanny island by forcing an uprise in Europe very far from England. Then Serbs were glorified by the medias and also allied politicians, because their one and only military commander General Draza Mihailovic made a sort of island of freedom in all nazi-occupied Europe. Then, in 1943, they diden't need Serbs any more, asking Chetniks to commit suicide for still beeing glorified. As Chetniks were reasonable people, they refused, then Churchill had his alibi to transfer support to communist partisans for who: the most Serbs were killed - the most they felt happy.
From 1943 on, Churchill was thinking of extending the UK influence in the Balkans, that's why he supported those who had the best probability to blindly obey him. The main reason was preventing Russia (USSR) to reach warm seas (Adriatic, though Mediterranean see) which is vital for developping trade and exports to the World (according to all Geopolitical rules in the last 150 years).
Even with Western Support, communist partisans coulden't take power in yugoslavia, until Soviet soldiers crossed Serbian borders in october 1944, gave freedom, shoulder to shoulder with Draza Mihailovic's Chetniks, to Serbian towns, then turned their arms toward Chetniks, arresting them and released power to communist partisans.
Churchill in 1945 tried to hide that he was betrayed by yugo-partisans, when he fled to an island on the Adriatic coast to meet tito for a few days, he met him the first day, and in the morning of the second, Churchill had to face the evidence: tito fled from their meeting in the early morning to meet soviet officials in Belgrade without informing Churchill about that. Hopefully for the West, as tito was all but legitimate, his dicatorship couldn't function without the Marshall plan that enslaved and destroyed the Serbian economy until nowadays.
Sorry, I coulden't cut the story shorter...