By itself, their declaration meant little. Yugoslavia hardly existed in practice, partitioned between the German Reich and its Hungarian, Bulgarian and Croat allies. The royal government, which in April 1941 left the country to continue the fight from exile (as did the governments of Czechoslovakia, Poland, France, and Greece, among others) had appointed General Mihailovich, a staff officer leading the guerrilla movement, their Minister of War and commanding officer of the Yugoslav Army in the Homeland. In addition to fighting the Germans, Croats, Hungarians, Bulgarians, Albanians and even some collaborators among the Serbs, Mihailovich's guerrilla also fought the Communist partisans, who emerged following the Nazi invasion of the USSR and made their priority to claim Yugoslavia for the socialist workers' revolution.
By late 1943, after Stalingrad and Kursk, it was clear that Germany would lose the war. That the Soviet tanks would show up was a question not of whether, but of when. Meanwhile, the Western Allies landed in Italy, forcing its surrender in September 1943.
That had multiple consequences for the war in Yugoslavia. Until then, the Italians were able to suppress the genocidal rampages of Croats and Albanians. Afterwards, they had a free hand and full German support, in exchange for Waffen-SS divisions made up of Albanians and Bosnian Muslims (Skenderbeg, Handschar, Kama).
The Communists did nothing to stop the atrocities. In line with their dogma, the Serbs were "oppressors", while the Croats and Albanians were the "oppressed" - so even though the Albanian leadership and the Ustasha were "reactionaries" and "fascists" in the Communist book, the mass murder and expulsion of Serbs were not objectionable as such.
To be fair, Communists weren't the only ones at the meeting in Jajce. Some of the "delegates" were pre-war politicians from opposition ranks: Croat separatists, Bosnian Muslims, and others generally sympathetic to the Communist platform of resurrecting Yugoslavia, but as a federation. If the Communists were the radicals, these "democrats" were their useful idiots.
Meanwhile, the Serbs in Communist ranks have by then so internalized the dogma of their own collective guilt for alleged "bourgeois imperialism", become so fanatical in their faith - and make no mistake, Marxism was a religion, though its deity was of this world - that they not only agreed to stand by while their families were being slaughtered, but to shift blame for the atrocities onto the designated "fascists," while the collectives that participated were actually rewarded. Thus arose the post-war Socialist Republic of Croatia, laying claim to Istria, all the Adriatic coast, Dubrovnik and western Syrmia, for example. Thus came about the "Autonomous province of Kosovo".
Why did the Communists believe that November 1943 was the right time to declare themselves the new rulers of a country they had yet to resurrect from under the Nazi heel? The Red Army was coming, but it would take them another nine months. Could the answer lie in the West?
In 1915, the Serbian Army and government retreated before the German, Austrian and Bulgarian invasion; the survivors reached Entente territory in Greece, and were deployed at the Salonica Front. In September 1918, the Serbs spearheaded the Entente attack and rolled up the front; six weeks later, they had not only liberated their homeland, but were approaching Vienna. The royal Yugoslav government hoped for a repeat performance, with an Allied landing along the Adriatic coast helping Mihailovic launch a general uprising. But the plans for an Adriatic Landing never went beyond the theoretical.
A day before the meeting in Jajce, Stalin met with Churchill in Tehran, and demanded the British switch their support from Mihailovich to Tito's Communists. Churchill wasted no time in agreeing. Supposedly, this is because Tito's men were "killing more Germans" - which was simply not true. But the fact that Stalin's demand and the meeting in Jajce were almost simultaneous suggests it was coordinated on the Communist part.
As for Britain's betrayal, it is a fact of history - only the motivations remain beyond conclusive explanation just yet. There are several theories to explain it, from secret Communist sympathizers in British intelligence (who did exist), to a story that young Churchill was roughed up by some Serbian officers for libeling the Serbs while he covered the Balkan Wars as a journalist. But the best explanation is probably the simplest: to London, the Serbs have ever been but an extension of the hated Russians, so Whitehall preferred a Croat-led Yugoslavia that would keep the Serbs under control. Interestingly enough, Hitler thought the same.
Another clue can be found in the decision of Jajce revolutionaries (calling themselves the "Anti-Fascist Council of People's Liberation of Yugoslavia, or AVNOJ) to ban the royal government from returning to the country. In sections 3 and 4, the AVNOJ leadership is tasked to "review all the international treaties and obligations" the royal government entered into, "for the purpose of nullification or approval", and declared all subsequent treaties made by the royals null and void.
This enabled both London and Washington to effectively confiscate the gold reserves the royals managed to take with them, as "payment" for all the military aid provided to both Mihailovich and the Communists. The remaining gold, hidden in Montenegrin caves, was discovered in 1943 by an enterprising Italian officer - who sent a small portion to Mussolini, gave the half of the remainder to Tito in 1944, and kept the rest for himself. Meanwhile, the Communists kept telling the people the "corrupt plutocrats" of the royal government stole all their gold. And while King Peter II died broke, Tito lived and died like a pharaoh.
In addition to throwing Stalin a bone - on account of the Red Army doing the bulk of the fighting in Europe - the Western Allies had a few more reasons to back Tito. For one, that avoided the sticky matter of the wartime Croatia. Horrific crimes of the Croatian state, backed by the Roman Catholic Church, had disgusted the Italians and unsettled even the Germans. How could anyone ask of the Serbs to re-create Yugoslavia with the Croats, after that? Easy enough: by having Tito denounce the Pavelic regime as "a handful of fascists," then rehabilitate Croatia as a federal republic in the new Yugoslavia. And while the Serbs had to continue apologizing for their existence - "oppressors," remember? - a top Croat official (Stevo Krajacic) was able to tell the families of Serbs murdered in the Jasenovac camp complex,"we killed too few of you here." (1968)
The suppression of Croat atrocities not only made Tito's Yugoslavia possible, it was also extremely useful for keeping the Church of Rome useful during the Cold War, as a tool of anti-Communism in places like Poland.
And so, on that November night in Jajce, a plan approved in Tehran was set in motion. Hitler had already unwittingly provided a template. Eighteen months later, when Soviet tanks drove the Germans out, Tito became the pharaoh of a reanimated Yugoslavia. Though the principal victims of Nazi invaders, and principal fighters against them, Serbs loyal to the king were persecuted, and even those who backed Tito found themselves third-rate subjects in their own country. Adding insult to injury, they were told this nightmare was the ultimate fulfillment of their historical dream of freedom.
Though both Tito and Yugoslavia are long gone, the nightmare endures. Seventy years later, it is high time for the sleeper to awaken.