In 1940,-41 Germany was able to enlist the support of Roumania, Hungary (the latter at the expense of the former), and Bulgaria, in the Balkans. Yugoslavia initially signed on with Germany, then soon switched sides and declared herself for the Allies.
Why did Yugoslavia do this? I have a recollection of a difference of opinion between the minor (seventeen year old) king, and his uncle, the regent. Was this the real cause, or were one or both men pawns in a larger struggle?
Was there a united Yugoslavia? There was a Yugoslavia, but it was split to Croatia (with Bosnia and Herzegovina), Serbia and Slovenia (which was again split for Germany/Austria Italy and Hungary).
I don't think you can say Yugoslavia switched the side, there were different parties. During the war, the Ustaše (Croatian ultra-nationalists) stayed with Nazi-Germany.
Signing the pact with Germany by Yugoslavia didn't take an easy path. It took months of negotiations and tweaking which finally ended up on March 25th, 1941 when the treaty was signed. Only two days later, well-known March 27th took place overthrowing the regent Pavle and bringing 17 year old King Peter to reign, at least formally. British intelligence had a high level of involvement in this event as they desperately needed more enemies of their main enemy - Hitler.
Only few decades before Serbs which constituted majority of Yugoslavian population at the time were fighting bloody war with Austro-Hungarians and Germany and it would have been quite unlikely for Serbs to take Hitler's side in the war. Weeks later Nazis bombed Belgrade and then penetrated into Yugoslavia with little or no resistance. While having great ovations while marching into capitals of Slovenia and Croatia, no such a treatment took place in Belgrade.
Once Yugoslavia was conquered by Nazis, there were many militant movements involved in the conflict. Just to name few major ones: Yugoslav Partisans, Royal Chetniks and Croatian Ustashe. As some movements collaborated with Nazis, others were fighting them, and some did both, it is hard to talk about Yugoslavia as one entity during the war.
As Hitler wanted to conquer the Soviet Union, he wanted to establish full control of southeast Europe. So he forced Yugoslavia to join the Tripartite Pact. That happened on 25th of March 1941. People in Yugoslavia were unhappy because of that decision. Also, it did not sit well with Great Britain because they lost another ally on east.
All of those were the reasons why the Chief of the General Staff, Dušan Simović, supported by Great Britain, performed a Yugoslav coup d'état on the 27th of March 1941.
Hitler was very angry, even though Dušan Simović stated that Yugoslavia wouldn't leave the Tripartite Pact. Hitler thought that was just buying time and attacked Yugoslavia on the 6th of April 1941. After 4 days there was declared the Independent State of Croatia under Nazi and Fascist control and on 17th of April, the rest of Yugoslavia capitulated.
