Seeing as the current theme of the week is the Balkans I thought I'd ask a long-standing question I've wanted to ask. Post-war Socialist Yugoslavia is definitely better known but I'm more interested in interwar Yugoslavia. What was the political system like and how did they keep the nations together? And how did it interact with it's neighbors on the world stage.
Also why was a monarchy established, especially a Serbian one, when wouldn't a republic similar to the ones in Poland and Yugoslavia be more favorable? It certainly wouldn't stoke the idea it was a plan for a Greater Serbia.
POLITICAL SYSTEM
The basis of all problems came in the very way Yugoslavia has been united. Serbia came out as a victor of the WWI. In parallel to the advancements of the Serbian army, Montenegro, Syrmia, as well as regions of Banat, Bačka and Baranya, declared direct unification with Serbia. The rest of Yugoslav area under Austro-Hungarian rule united into practically imaginary State of Slovenes, Croats and Serbs. The reason for existence of this imaginary country was the attempt to preserve as much autonomy vis-a-vis future central authority as possible. However, due to the fact that it was Serbian weapons that decided the war in Yugoslav territory, their attempts to preserve autonomy failed. Serbian political elite wanted a centralist state with agrarian reform (ending feudalism) and democratic principles (no census in electorate, for example), that was extremely unpopular with leading politicans from Austro-Hungarian era. Now, Croats and Slovenes made many works, proving that such policies led to creationg of Greater Serbia of a kind; however, the Serbs from present-day Vojvodina and other ex-KuK regions were also discontent with centralism and democratic policies.
Yugoslavia was made of three constituent nations: the Serbs, the Croats and the Slovenes. Other Slavic nations, like the Muslims/Bosniaks and the Macedonians weren't recognized as such. In order to decrease compactness of "minorities" like Albanians or rebellious members of constituent nations like the Macedonians, government used method of colonization, mainly of Serb families, but also of other constituent nations (mainly Croats). In that way a problem of agrarian overpopulation in core Serb lands was being solved. Needless to say, that led to practically mini-wars with Albanian minority in Kosovo-Metohija and West Macedonia, as well as with Bulgarian/Macedonian minority in rest of Macedonia. Bulgaria and Italy (through Albania) actively used this resentment to make widespread terrorist actions in those regions.
Now, as one probably knows, Yugoslavia was extremely heterogeneous. The differences weren't only national or religious; regions varied in development greatly as well: present-day Slovenia had ~8% of illiterate population, while present-day Macedonia had ~88%. And Yugoslavia didn't succeed in ironing these differences. For example, the pupils in elementary schools didn't have same history textbooks: Croat children were learning from old, Austro-Hungarian textbooks, while Serb childrean were learning from old, Serbian textbooks.
The other big problem was the opposition towards royal absolutism. At the beginning, barely anyone had problem with Yugoslavia being a parliamentary monarchy. But King Alexander I made it clear even during the WWI (when he was still a regent) that he had no intention of ruling like British monarchs. The crisis went on as the Croat Republic Peasant Party and the Communist Party of Yugoslavia got 3rd and 4th place in first elections. This was made worse when leader of the most popular Croat party (CRPP) got assassinated by a Serb politician in the middle of the Parliament. Now the party included extremely nationalist Croat elements, and the new leader, Vladko Maček, espoused the politics of harassing local Serbs.
King Alexander I then started with the experiment of forcibly uniting Yugoslavs, prohibiting all national insignia, parties and such. Of course, such a move produced result of the king finding himself assassinated by Croat-Macedonian coalition in 1934. Vladko Maček gathered around his party every discontent political factor: the Slovenes, the Austrian Serbs, the left-wingers in 1932. In the 1935-1939 period, PM Milan Stojadinović successfully kept the Croats in check by allying Croat allies (Germany, Italy, Hungary). However, those policies proved impossible and in 1939 the Banate of Croatia was formed out of Croatian historical lands. The Banate of Croatia practically became independent. Already in those years first trails of ethnic violence against minorities were shown. Up to this day it remains controversial how the leadership of the Banate was connected to Ante Pavelić.
FOREIGN RELATIONS
After the end of the WWI, Yugoslavia found itself encircled by losers of the said war (Austria, Hungary, Bulgaria, de facto Italy). The only ally, France, was distant and uninterested in Yugoslavia, save for extracting as much money as possible in the name of debts. Out of all enemies, Italy was the most dangerous one: it declared itself enemy in 1918 already, and already in 1920 Yugoslavia had to fight Italian demands on the Adriatic coast. At the same year, Italy made its first attempt to create anti-Yugoslav coalition with Albania and Bulgaria in it. Rise of the fascist in Italy brought to bigger conflicts, when Italy practically annexed the town of Rijeka (Fiume). Italy forced Yugoslavia in 1925 and 1928 to give various concessions to Italy in the Adriatic sea, using internal conflicts. Since 1928, Italy has started with attracting Austria, Hungary and Bulgaria in anti-Yugoslav coalition.
In 1933 Germany and Italy signed alliance. In 1934 King Alexander I of Yugoslavia was assassinated in France. That resulted in fall of Franco-Yugoslav alliance. In 1935 Germany became the main trading partner of Yugoslavia, therefore expelling her from the French sphere. The de facto leader of Yugoslavia in post-1935 period, PM Milan Stojadinović, openly espoused politics of reconciliation with Italy and Germany). However, almost every other party in the country protested Stojadinović's policies, wanting to keep traditional alliance with France.
Yugoslavia was in alliance with two other countries in French sphere, Czechoslovakia and Romania (The Little Entente), since 1920. These countries were supposed to be united against common enemies, losers of the WWI in the eastern Europe. However, Stojadinović brought this alliance to end in 1937, when he, without informing his allies, signed the agreement of eternal friendship with Bulgaria (the turn in relations came after Bulgarian coup d'etat in 1934). In 1940 Yugoslavia signed a similar pact with Hungary, which was made possible with German economic influence in both countries (in 1934, when King Alexander I was murdered, Hungary was accused as one of conspirators, and 2.700 Yugoslav Hungarians were exiled).
German investments have taken the shape of economic offensive since the Anschluss: 46.7% of Yugoslav industry, 40% of artisan production and 30.6% of banking capital was in German hands. As well as 30% of fertile land in Vojvodina, the most fertile land of Yugoslavia.