The actions of the Black Hand terrorist group not only brought down their own rival but the whole world. The Europe kingdoms and empires were destroyed in World War I. The idea of monarchies being the standard form of government went out the window. The United States rose to international power because its lack of causalities. American President Woodrow Wilson pressed republican ideals on Europe. The Communists saw their opportunity to after the war. Sometimes they succeeded with the rise of the Soviet Union, fall of Ukraine; while other times they failed like in Poland, various German states, and Ireland.
Reactionaries and regressive progressives (fascists who sought to move Europe forward while glorifying some idealized past) tried to fight the political-cultural tied. They fought World War II and failed, taking fifty million lives with them. Afterwards the two successful ideologies out of World War I (Republicanism and Communism) fought their own Cold War. Republicanism finally won out for the most part.
At the end of the Cold War many thought we had finally broken away from history. Sarajevo helped bring us back to history though. A civil war in Yugoslavia and the siege of Sarajevo made people realize war is still possible in the "civilized" world. NATO had to be activated to stop the fighting. Before NATO's entry people wondered if the alliance was needed any more and should be done away with. The war proved the continued importance of the international coalition which is fighting in Afghanistan today.
The assassination of the archduke outside a cafe started the geopolitical trend of the twentieth-century. The fall of Yugoslavia showed how things change, how things stay the same. While there are many important historical places of the twentieth-century, Sarajevo though is the key landmark of the time.