1941: Enraged by Belgrade's rejection of the Tripartite Pact, Adolf Hitler orders Unternehmen Strafgericht (Operation Punishment), the attack on the Kingdom of Yugoslavia. Italy, Hungary, Romania and Bulgaria join the invasion. By April 10, Croat troops had mutinied and the "Independent State of Croatia" was established, eagerly welcoming the Germans. By April 18, the war was over and Yugoslavia had ceased to exist, partitioned by the invaders.
What followed was four years of uprisings (by royalists and Communists), brutal reprisals by the occupiers, and a genocide of Serbs, Jews and Roma by the "Independent State of Croatia." Whether the Yugoslav insurgency really disrupted the German war effort to any great extent is debatable, but the fact remains that Hitler's whim delayed the planned invasion of the USSR by five weeks.
1992: Washington and countries of the EEC (precursor to the EU) recognize Bosnia-Herzegovina as an independent state. The recognition was requested by the Muslim-dominated regime of Alija Izetbegovic, seeking to trump the objections of the country's Serbs. Just over a third of the country's population, the Serbs supported staying in Yugoslavia, but were willing to accept an independent Bosnia if it were organized on the Swiss model (a confederation of ethnic cantons). Bosnian Croats backed Izetbegovic's declaration of independence, but also sought territorial autonomy; units of Croatian Army were already present in many parts of Bosnia, skirmishing with the retreating Yugoslav federal army and Serb militias.
In February of 1992 it seemed that a compromised had been achieved under the aegis of the EEC, with all three groups agreeing on a proposal submitted by Portuguese diplomat Jose Cutilheiro. However, in March Izetbegovic reneged on the agreement, following a consultation with the U.S. Ambassador to Yugoslavia, Warren Zimmerman. Convinced (erroneously) that he had Croatia's backing, confident in U.S. support, willing to sacrifice as many lives as necessary to achieve his goal of Muslim-dominated Bosnia, Izetbegovic simply refused to make any deals with the Serbs. Recognition of his regime closed the door on all political and diplomatic avenues of resolving the Bosnian conundrum; Western policymakers claimed it was supposed to prevent a war; in fact, it made war inevitable.