The history of nations is mostly characterized by ethnic and racial uniformity, not diversity.
Most national boundaries reflected linguistic, religious, and ethnic homogeneity. Until the late 20th century, diversity was considered a liability, not a strength. Countries and societies that were ethnically homogeneous, such as ancient Germanic tribes or modern Japan, felt that they were inherently more stable and secure than the alternative, whether late imperial Rome or contemporary America.
Many societies created words to highlight their own racial purity. At times, “Volk” in German and “Raza” in Spanish (and “Razza” in Italian) meant more than just shared language, residence, or culture; those words also included a racial essence.
Even today, it would be hard for someone Japanese to be fully accepted as a Mexican citizen, or for a native-born Mexican to migrate and become a Japanese citizen. Many cultures reflected their suspicion of diversity by using pejorative nouns for the “other.”
In Hebrew, the “goyim” were all the other non-Jewish nations and peoples. “Odar” in Armenian denoted the rest of the world that was not ethnically Armenian. For Japanese, the “gaijin” are those who by nationality, ethnicity, and race cannot become fully Japanese. In 18th-century Castilian Spain, “gringo” meant any foreign, non-native speakers of Spanish.
The Balkan states were the powder kegs of 20th-century world wars because different groups wanted to change national boundaries to reflect their separate ethnicities. The premise of Nazi Germany was to incorporate all the German “Volk” into one vast racially and linguistically harmonious “Reich” — even if it meant destroying the national borders of Austria, Czechoslovakia, and Poland.
The constitution of Mexico unapologetically predicates national immigration policies on not endangering Mexico’s ethnic makeup.
The United States steadily evolved to define Americans by their shared values, not by their superficial appearance. Countries, ancient and modern, that have tried to unite diverse tribes have usually fared poorly. The Italian Roman Republic lasted about 500 years. In contrast, the multiracial Roman Empire that after the Edict of Caracalla in AD 212 made all its diverse peoples equal citizens endured little more than two (often violent) centuries. Vast ethnically diverse empires such as those of the Austro-Hungarians, the Ottomans, and the Soviets used deadly force to keep their bickering ethnic factions in line — and from killing each other.
Modern states such as multicultural or multi-tribal Rwanda, Iraq, and Lebanon have often proved deadly failures. Europe is trying to emulate the multiracial but unified culture of the United States. But the European Union may well tear itself apart trying to assimilate millions of disparate migrants who are reluctant to fully assimilate.
America is history’s exception. It began as a republic founded by European migrants. Like the homogenous citizens of most other nations, they were likely on a trajectory to incorporate racial sameness as the mark of citizenship. But the ultimate logic of America’s unique Constitution was different. So the United States steadily evolved to define Americans by their shared values, not by their superficial appearance. Eventually, anyone who was willing to give up his prior identity and assume a new American persona became American. The United States has always cherished its “melting pot” ethos of e Pluribus Unum — of blending diverse peoples into one through assimilation, integration, and intermarriage.