The Slavs were divided by this war. Slovenia, Croatia, and Bosnia fought on the side of the Central Powers (Austria-Hungary and Germany), while Serbia and Montenegro fought on the side of the Allies. Later in the war, Serbia's longtime enemy, the Ottoman Empire, joined the Central powers, as did Bulgaria.
Croat, Macedonian, and Albanian extremists sought to topple the kingdom. They recieved support from the Bulgarian, Hungarian, and Italian governments.
The Croatians campaigned for independence and socialism. To boost their own ethnic power, the Serbs took over the government by inserting Serbs into key positions. Non-Serbs were discriminated against.
In the 1920s, both the Interior Minister and King Aleksandar were targets of assasination attempts. King Aleksandar lived, but the Interior Minister was not so fortunate. As the assasination attempts took place, communism was on the rise, and corruption reigned supreme. With complete anarchy on the horizon, in 1929 King Aleksandar dissolved all democracy and became "temporary" dictator of the Kingdom.
Aleksandar's efforts to bring the ethnic groups together backfired, and they were just pushed further apart. In 1931, a limited democracy was established. Soon after, the "Balkan Entente" was created. It was a limited defense pact between Yugoslavia, Romania, Greece, and Turkey. The reason for the pact was that, at this time, the waters of fascism were moving towards high tide in Germany and Italy.
In 1934, Aleksandar was assasinated. All feared that Yugoslavia would disintegrate, and Italy would invade. Aleksandar's cousin, Prince Pavle, formed a three man regency because there was not one man ready to handle the job by himself. This new government was more permissive of dissenters.
The lenincy towards dissenters savd the life of many a Croat, who were constantly trying to break free. They were not allowed to secede ,because, if they left, Yugoslavia would be geographically divided, and the Italians could easily invade.
In January, 1937, with Axis expansion looming to the north as Hitler gobbled up Austria, Yugoslavia and Bulgaria signed an eternal friendship pact. In March of that year, Yugoslavia signed a frienship treaty with Italy, in the hope of defending itself from takeover. That hope was in vain. Even though Yugoslavia was officially and diplomatically friendly with Bulgaria and Italy, popular support went to England and France.
As it became apprent that Yugoslavia itself was shifting towards fascism, several members of the government were replaced to make sure that did not happen.
In 1939, German forces edged towards Yugoslavia's northern border, and Italy occupied Albania. Alarmed, Yugoslavia secretly shipped its gold reserves to England and France. When discovered, this move angered the Axis.
With invasion imminent, now more than ever it was important to maintain national unity. In a move to achieve this, the Croats were appeased by being given more self government; but instead of satisfying their thirst for independence, this made them crave it all the more. This caused the Bosnian Muslims to demand autonomy. Yugoslavia was splintering directly in the footsteps of the Axis advance.
When France surrendered in 1940, Yugoslavia lost its most dependable ally. To avoid destruction, Yugoslavia was forced to sign the Tripartite Pact with Germany, which guaranteed that the Germans would respect Yugoslav soveriengty. After huge anti-Axis demonstrations int he streets of Belgrade and other major cities, Hitler bombed Belgrade and invaded Yugoslavia anyways.