The story of Yugoslavia is a cautionary tale and a warning for those who underestimate the strength of nationalism.
Yugoslavia was an artificial country, created after World War I from Serbia, Montenegro, and much of Austria-Hungary. It was a monarchy run by the same royal family that ruled Serbia before the war. It was occupied by Nazi Germany and Fascist Italy during World War II. Yugoslavia was liberated not by the Soviet Union, but by its own partisan forces. Therefore, there were no Red Army troops in Yugoslavia to force the country to become a satellite state of the Soviet Union like the other countries of Eastern Europe.
Yugoslavia was held together by the unifying figure of Josip Broz Tito. He was an ethnic Croat, not the largest ethnic group in the country, but he was still respected by the various peoples of Yugoslavia for his role in liberating Yugoslavia from German and Italian occupation. While Yugoslavia was communist, Tito remained independent of Joseph Stalin and led Yugoslavia his own way. Tito was first allied with Stalin but broke with him in 1948. Yugoslavia was an important figure in the non-aligned movement during the Cold War. Tito promoted "Brotherhood and Unity" and suppressed nationalism, sometimes by force.
Tito died in 1980. In the 1980s, the country's economy declined, and nationalism began to rise. The country broke up in 1991 during the fall of communism in Eastern Europe. The country had been divided into six republics and two autonomous provinces. The republics of Slovenia, Croatia, Bosnia and Herzegovina, and Macedonia all seceded, leaving Serbia and Montenegro as the remnants of Yugoslavia.
Serbia was led by Slobodan Milošević. Milošević wanted to create a Greater Serbia, where ethnic Serbs living outside Serbia would be incorporated into their country. Specifically, he wanted the regions of Bosnia and Croatia that were majority Serbian. Serbs in Croatia didn't want to live under Croat rule, because during World War II, the Croats allied with the Nazis and fascists and committed many atrocities against the Serbs. They tried to secede from Croatia. Croatia objected to this, to which Milošević responded that if Croatia could secede from Yugoslavia, then Serbs living in Croatia could also secede.
Serbs in Bosnia also tried to secede and conquered much of the country. The Muslims and Croats in Bosnia united to fight the Serbs. After NATO bombed Serbian-controlled areas in Bosnia, they agreed to negotiate. The Dayton Agreement was signed in 1995, which led to the end of the war. Bosnia became a union of two entities, the Republika Srpska and the Federation of Bosnia and Herzegovina. A few years later, fighting broke out in Kosovo. Kosovo was majority ethnic Albanian, but Serbia still claimed the province, because it was part of the Kingdom of Serbia and the province of Serbia in Yugoslavia, and it had much historical significance. It was the site of the defeat of Serbia by the Ottoman Empire in 1389. NATO bombed Serbia, which led to the country withdrawing troops from Kosovo. Gradually, peace returned to the region. Montenegro became independent from Serbia in 2006, and Kosovo declared independence in 2008, although Serbia, Russia, and many other countries don't recognize it.
The trend since 1914 has been countries breaking up, not uniting. Austria-Hungary broke up after World War I, its territory becoming part of seven countries, some of which were new. After the fall of communism, Yugoslavia broke up, as was detailed previously. So did Czechoslovakia, which peacefully broke up into the Czech Republic and Slovakia in 1993. So did the Soviet Union itself, which in 1991 broke up into 15 different countries based on the old Soviet socialist republics. This caused ethnic problems, because there were large minorities of Russians in the Baltic states, Ukraine, and Kazakhstan. These problems flared up in 2014, with the unrest in Ukraine and Russia's annexation of Crimea.
Ethnic conflict in multiethnic countries is not limited to Europe. Most modern African countries were created by European colonial powers and have no ethnic majority. This has led to ethnic warfare, one example being the Nigerian Civil War of 1967 to 1970, where the Igbo people tried to secede as the nation of Biafra but were defeated. Most modern Middle Eastern countries were also artificially created by European powers after World War I. This has led to violence in Iraq among Shia Arabs, Sunni Arabs, and Kurds. This has also led to ethnic and religious tensions in other countries such as Syria and Lebanon.
It certainly seems that diverse countries tend to be unstable. That being said, what lies ahead for the most diverse country in the world, the United States?
It is highly likely that the United States will break up sometime in the near future. Since the 1960s, the United States has become more racially heterogeneous and more politically polarized. The right and left have grown increasingly farther apart and see each other not as fellow Americans, but enemies. This polarization has accelerated since the presidential election of Donald Trump in 2016. Americans used to mostly have the same religion, Christianity, and now they don't. Many are irreligious or are members of other religions. The immigration of large numbers of people from Latin America and Asia since the Hart-Celler Act of 1965 has transformed the United States. As a result of this mass immigration, white people are projected to become a minority in the United States in 2042. No other country has undergone such a rapid demographic transformation in such a short period of time.
Many racial nationalists want a piece of the United States for themselves. Some Mexican nationalists want the Southwest to become a part of Mexico again or to become an independent country called Aztlán. Some black nationalists want the Deep South to become an independent all-black country, believing that black Americans have a different identity from other Americans because they were enslaved and therefore deserve their own country. Some white nationalists, especially in the Alt-Right movement, want one part of the United States to become an "ethnostate" where only white people live, the most common proposal being the Pacific Northwest.
Some might argue that the number of people in the United States who want to see the country break up are small in number, and therefore it is unlikely to happen. This is true, but the number of things that unite us as a country are becoming fewer, and the number of things that divide us as a country are growing. This trend shows no signs of stopping or slowing down.
A country without a common sense of nationhood won't last. If the United States were racially diverse but politically united, it could survive. If the United States were politically divided but racially homogeneous, it could survive. But if the United States is both racially diverse and politically divided, it will not survive.
If this country does break apart, will it happen peacefully as in Czechoslovakia, or violently as in Yugoslavia? Time will tell, but if our Civil War is any indication, unfortunately, it will likely be more like Yugoslavia. Let us hope this is not the case.