ENCYCLOPEDIA BRITANNICA: 15th edition, (year: 1986), Volume 29, Macropedia, page 1075
Entry: Yugoslavia, World War I

Quote:

[Already] on December 7, 1914, the Skupstina [Serbian Parliament] had unanimously resolved that "Serbia's only aim is the liberation and union of all our Serb, Croat, and Slovene brethen." But the policy of the Western Allies, based on expediency, ran counter to Serbia's aim and even to Serbia's integrity as established in 1913. Without the knowledge of Serbia, the Entante concluded the secret treaty of London with Italy in April 1915, made concrete proposals to Bulgaria in May, and forbade a Serbian declaration of war on Bulgaria in September, when that state was on the verge of attacking Serbia... Bulgaria attacked on October 11; and by November 25, Serbia, overcome by combined German, Austro-Hungarian, and Bulgarian forces, was partly occupied by Austria-Hungary and partly by Bulgaria. The retreat of [the Serbian] king [Peter I], government, Skupstina, and masses of the people with the Army across Albania stands out as one of the great exploits of any war.

With such "allies" who needs enemies?

Formation of Yugoslavia

Quote (from the same source as above):

Corfu [an island in Grece] became the seat of the Serbian government. Reequipped, the Army joined the Allies at Salonica...

The government, the opposition, and the Yugoslav Committee on July 20, 1917, signed the Corfu declaration, affirming that Serbs, Croats, and Slovenes constituted one nation and demanding union in a constitutional, democratic, and parlimentary monarchy. In 1918, when the Allies launched the Salonica offensive, the Serbs routed Bulgars at Dobro Polje and liberated Prilep... Bulgaria capitulated. Nis", Prizren and Mitrovica were liberated, and the victorious Serbian Army entered Belgrade on October 31. The war had cost Serbia 23 percent of its population.

Every second Serbian male died in World War One. Whole generations were wiped out.

This is how one of many Serbian village graveyards look like. Monuments to young Serbian soldiers.

Quote (the same source, page 1076):

On November 24, 1918, at Zagreb in Croatia, the National Council of Slovenes, Croats, and Serbs, representing the Slavs of Dalmatia, Croatia, Slavonia, Slovenia and Bosnia, proclaimed the union of their territories, hitherto all subject to Austria-Hungary, with Serbia and Montenegro in one State of the Serbs, Croats, and Slovenes. It then sent a delegation to invite Serbia to adhere to this union. On November 25, however, a National Assembly of the Serbs of Vojvodina, at Novi Sad, voted for the immediate union of their country with Serbia; and on November 26 a Montenegrin Grand National Assembly, at Podgorica, voted for union with Serbia. Serbia's area was increased greatly in the few days before Alexander, as regent for Peter I, on December 1, 1918, in Belgrade, accepted the invitation from Zagreb and so brought into being the Kingdom of Serbs, Croats, and Slovenes...

In 1929,... the kingdom officially took the name of Yugoslavia.

Note that in the above text Dalmatia, Croatia, Slavonia were mentioned as different entities. It was Croatian Communist Tito's invention to assemble all of these different entities into one "Socialist Republic of Croatia".

Note that Croats and Slovenes have asked the Serbs, the victorious side in the World War to accept them into the union. The Serbs did NOT form "Great Serbia", they treated Croats and Slovenes as equal. This, despite the fact that many Croats and Muslims fought in the enemy [Austro-Hungarian] uniform, and many of them committed atrocities on the defenceless population of Serbia during its occupation by the enemy troops. The Serbs were ready (may be too ready) to forgive.

The Serbs have achieved their war aim. They have liberated other South Slavs. They did it at tremendous price to their population. The Serbs have invested TWO Serbian states into Yugoslavia. Croats and Slovenes invested nothing.

In Serbo-Croatian "Yugo" means South. Yugoslavia means: The country of the South Slavs.