... In multiracial, multiethnic, and multi-religious societies—such as contemporary India or the Middle East—violence is the rule in the absence of unity. Even the common banner of a brutal communism could not force all the diverse religions and races of the Soviet Union to get along. Japan, meanwhile, does not admit many immigrants, while Germany has welcomed over a million, mostly young Muslim men from the war-torn Middle East.

The result is that Japan is in many ways more stable than Germany, which is reeling over terrorist violence and the need for assimilation and integration of diverse newcomers with little desire to become fully German.

History offers only a few success stories when it comes to diversity. Rome, for one, managed to weld together millions of quite different Mediterranean, European, and African tribes and peoples through the shared ideas of Roman citizenship ( civis Romanus sum ) and equality under the law. That reality endured for some 500 years.

The original Founders of the Roman Republic were a few hundred thousand Latin-speaking Italians; but the inheritors of their vision of Roman Republican law and constitutionalism were a diverse group of millions of people all over the Mediterranean.

History’s other positive example is the United States, which has proven one of the only truly diverse societies in history to remain fairly stable and unified—at least so far. Although the Founders are now caricatured as oppressive European white men, they were not tribal brutes. The natural evolution of their unique belief that all men are created equal is today’s diverse society, where different people have managed, until recently, to live together in relatively harmony and equality under the law.

Unlike present-day Mexico, China, or Japan, America never developed a fixed idea, either culturally or formally in its written constitution, that race or religion de facto defined citizenship. Instead, an imperfect America was always being reinvented in dogged pursuit of the Founders’ promise of equality and the toleration of difference.

Despite a Civil War that took over 600,000 lives, years of oppression and segregation, dozens of major riots, and thousands of court cases and legislative fights, our American exceptionalism held that America alone could pull off the bizarre idea that diverse peoples could eventually live together as a single people in brotherhood.

But the American experiment is not static, nor is it settled. The nation’s racial, ethnic, and religious diversity is by nature volatile, and prone to exploitation by demagogues and opportunists.

A diverse America requires constant reminders of e pluribus unum and the need for assimilation and integration. The idea of Americanism is an undeniably brutal bargain in which we all give up primary allegiance to our tribes in order to become fellow Americans redefined by shared ideas rather than mere appearance.

Unfortunately, there are increasing signs that our political, religious, ethnic, and racial diversity is overwhelming our shared but fragile notion of national unity. Growing geographical separation into blue coastal liberal states and red interior conservative counterparts is starting to mimic the North-South regional divide of the Civil War, a split in national geography that is fueling political differences. ...