In a single generation, between 1980 and 2007, more than 10 million people migrated, legally or illegally, from Mexico to the U.S. Today there are more than 12 million Mexican-born people in the U.S. and millions of American children who are their offspring—amounting to almost 10% of the nation’s population. That is exponentially larger than in 1970, when there were less than one million Mexican-born people in the country, or 1980, when there were two million. The Mexican migration, and the similarly large migration of others from the rest of Latin America, has in just one generation reshaped the nation. Hispanics have replaced blacks as the largest officially recognized minority group.

Needless to say, this transformation hasn’t gone unnoticed in our politics, especially in the border states most affected by the influx. Groups like the “minutemen,” self-appointed guardians of the U.S. border, may no longer hold the spotlight, but the issue remains tense, as suggested by the iffy prospects on Capitol Hill of the latest attempt at “comprehensive immigration reform.” Many Americans still worry that, with the profound shift in the country’s ethnic composition over the past several decades, the U.S. is well on its way to flying apart.

None of this should come as a surprise to a student of American history. But for perspective, it is helpful to recollect that the conflicts produced by previous surges of migration resulted in much worse strains. More than that, in the process of dealing with these strains, Americans have developed a capacity and a habit of accommodating and uniting citizens with very serious and deep differences. Going back to the Founding Fathers—with their formula of limited government, civic equality and tolerance of religious and cultural diversity—each new surge of arrivals has been greeted as a crisis without precedent, only to disappear with unexpected speed as the nation faced new challenges.

This was apparent in the surge of migration of the Scots-Irish from Northern Ireland and the Scottish Lowlands to the largely unsettled lands along the Appalachian chain in the dozen years before the American Revolution. Unlike the first settlers of the seaboard colonies, the Scots-Irish weren’t motivated by some mixture of religious and political beliefs, nor did they arrive subject to varying degrees of coercion, like black slaves and white indentured servants. These were fighting peoples, moving from one violent fringe of the British Empire to another, seeking places where they could live in liberty, at peace with their neighbors if possible, but always ready to fight fiercely if attacked.

For two generations in the early republic, the Scots-Irish fought to expand their zone of liberty to the southwest. Their archetypal leader was Andrew Jackson. Born in the Carolinas two years after his parents sailed from Northern Ireland, he settled in Tennessee, was elected to the House at age 29 and the Senate a year later. As a general, he ousted the Creek Indians from what is now Alabama and south Georgia and led Americans to victory over the British at New Orleans. He played a role in the U.S. acquisition of Florida and Texas and, through his support of his protégé James K. Polk in the 1844 election, opened the way for the Mexican War and the acquisition of California.

Meanwhile, New England Yankees, pent up in their homogeneous colonies for nearly 200 years by French-speaking Quebec and the British-Iroquois alliance in New York, suddenly surged westward over two generations, creating replicas of New England in upstate New York, northeast Ohio and southern Michigan; they also founded Chicago. The Yankee diaspora developed a vigorous evangelical Christianity that stressed good works and economic prosperity. And they spawned a series of reform movements—women’s rights, prohibition of alcohol, abolition of slavery.

Southern planter grandees also were moving westward, transporting their slaves in chains from the Atlantic seaboard to the Mississippi Valley, building what historian Robert Fogel tells us was a hyperefficient and wealthy Cotton Kingdom. Both the Yankees and the planters coveted territory farther west, and the issue of slavery in the territories split the Whig and Democratic parties and led to the formation of the Republicans as a purely Northern party. A map of the counties carried by its 1856 nominee, John C. Fremont, almost exactly tracks the New England diaspora. To that Abraham Lincoln added votes from Germans and others and carried every nonslave state but one and was elected president. The result was the Civil War.

What followed was proof that migrations aren’t just responses to economic incentives. The Northern victory resulted in confiscation of the wealth of the slave-owners and a Southern economy in which wages were less than half of those in the industrializing North. Yet, in the three generations from the Civil War to World War II, only a trickle of Southerners migrated north. There was no ocean to cross, no health screening to pass, yet in the years when more than 30 million Europeans migrated to the U.S., only about one million black and one million white Southerners migrated to the North.

In the early republic, there were few immigrants from overseas. Any screening was done by state governments, and the federal government didn’t start counting newcomers until 1820. Thus it was a considerable surprise when large numbers of Irish Catholics began arriving. Fleeing the potato famine, they came in huge numbers—1.5 million between 1845 and 1860—to a nation with just 20 million inhabitants (at the beginning of this period) and a deep suspicion of Catholics. This was the first great surge of migration into America’s large cities, especially Boston, New York and Philadelphia; few Irish wanted to go back to farming. Most had few skills, and many in the first years spoke only Gaelic. They tended to work as laborers and had high rates of crime and violence.

The Irish also had a knack for politics. They made their way upward in the nascent urban political machines, just as their more verbally skilled men and women made their way upward through the hierarchy of the Catholic Church. The Irish migration continued for two generations, and Irish-Americans put their stamp not only on popular politics but on popular culture. As the late Daniel Patrick Moynihan pointed out, American popular music and comedy a century ago had a distinctive Irish cast to it. It was George M. Cohan, son of Irish immigrant vaudevillians, who wrote “Yankee Doodle Dandy” and “You’re a Grand Old Flag.”

From the 1840s to the 1890s, there was another surge of migration, this time from Germany. Some Germans stayed in New York and Philadelphia, but most ventured west, to big German communities in the inland cities of Cincinnati, Louisville, St. Louis and Chicago. As time went on, more went to the Old Northwest—Wisconsin, Iowa, Minnesota, the Dakotas. German language newspapers and German singing and orchestral societies dotted this part of America. German education models—kindergarten, the research university—were widely adopted, as well as the German Christmas tree. German culture was nastily suppressed during World War I. But Germano-Scandinavian America has remained, as it was then, the most pacifist, isolationist and dovish part of the U.S.

The presence of so many Irish Catholics and Germans, Catholic and Protestant, seemed to threaten American culture. Heavily Democratic, Irish Catholics became a majority in the Yankee citadel of Boston; in response, the Massachusetts legislature decreed that the city’s police chief would be appointed by the (usually) Yankee Protestant Republican governor. In Cincinnati, St. Louis and Milwaukee it sometimes seemed that politics was conducted as much in German as in English, and German voters were a target group. One reason the Republicans nominated Abraham Lincoln in 1860 was that he rejected nativist and prohibitionist policies favored by some in his party.

But the U.S. had an inbuilt capacity to accommodate and assimilate outsiders. The Founding Fathers knew they were creating what many now call a multicultural nation. They knew that the 13 colonies had diverse religious and ethnic origins—Puritan New England and Anglican Virginia, Quaker Pennsylvania and Dutch Reform New York. So the Framers of the Constitution provided that the U.S., unlike Britain, would have no religious test for federal office. And the drafters of the First Amendment provided that the federal government could make no law regarding an establishment of religion—which meant leaving alone established churches in the states (Massachusetts’ lasted until 1833). The work of assimilation was left to the states as well, and it is significant that the states with the largest immigrant inflow—Massachusetts and New York—were among the first to pioneer universal public schools, where children were encouraged to understand and respect a common civic culture.

The project of assimilation found new challenges after the immigration station on Ellis Island opened in New York Harbor in 1892. At just that moment, amid economic distress, the flow of immigration shifted. A new surge began, from eastern and southern Europe. These were people who, in some sense, were second-class citizens in their home countries—Poles, Jews, Czechs, Slovaks, Slovenes and Serbs from the multiethnic Austro-Hungarian, Russian and German empires, as well as southern Italians from an Italy dominated by quite different northerners.

As a percentage of the pre-existing population, this surge of migration was among the largest in U.S. history. And these immigrants posed special problems. They came from polities where representative government, civic participation and guaranteed liberties were weakly (or not at all) rooted. They spoke languages that were further from English than German or the Scandinavian tongues.

The elites of 100 years ago responded with aggressive programs of, in Theodore Roosevelt’s word, Americanization. Public schools taught American values and made sure that immigrants’ children could read and write English. Henry Ford ran citizenship classes of the immigrant masses who worked in his giant factories.

The Ellis Island immigration ended suddenly, with the outbreak of World War I in 1914. It resumed after the war but was cut off by the immigration act of 1924, which assigned country quotas reflecting the ancestral origins of the Americans in the pre-Ellis Island 1890 Census. In any case, immigration (except for political refugees) would have tapered off in the Depression decade of the 1930s.

But what really assimilated foreign-born Americans and reunited the American North and South was World War II. It was an annealing event, soldering together different American ores, in a way that nothing short of total war can do. Some 16 million Americans served in the military (the proportionate number today would be 38 million). Servicemen and defense workers were sent all over the nation. The military remained racially segregated, but the war experience raised the argument that people who risked their lives for the nation should be treated equally when they came home, an argument that persuaded Presidents Truman and Eisenhower to integrate the military, now the most racially equal part of society.

The war triggered a great surge of migration, with one-third of American blacks moving from the rural South to the urban North in the single generation between 1940 and 1965. Widely circulated black newspapers hailed the North as a promised land, where blacks would be free to pursue dreams and escape a nightmare. But in the middle 1960s, with the end of legal segregation (and the widespread adoption of air conditioning), the South was suddenly less of a nightmare and, with urban riots and continued residential segregation, the North less of a dream. The northward black migration suddenly ended—another phenomenon almost no one predicted.

What lies ahead for our migrant nation? The housing collapse seems to have had a disproportionate effect on Hispanics, especially those of Mexican origin. Examination of the county foreclosure rates in the 2007-10 period suggests that one-third of those who lost their houses were Hispanics. And with Mexico’s economy now growing faster than ours, America looks less inviting and Mexico less forbidding. AsMitt Romney awkwardly noted during the recent campaign, many Mexicans who had come to the U.S. have chosen to “self-deport.” Robust economic growth in other Latin American countries—Colombia, Peru, Guatemala, Dominican Republic—could also lower Latin immigration, and new arrivals from these countries are likely to have higher skill and education levels than previous waves from south of the border. Assimilation seems to be proceeding, but less vigorously than in the first half of the 20th century, and there is no event on the horizon with the annealing capacity of World War II.

The most recent data show that, for the first time since the Chinese immigration to Gold Rush California, more immigrants are coming to the U.S. from Asia than Latin America. Asian immigrants on average have been more upscale than Latin immigrants. Thus the prospect is for more high-skill immigration, especially if Congress has the wisdom to tilt immigration law more in that direction, as Canada and Australia have done. But prediction is risky about a subject that, as history shows, has featured one unpredicted episode after another. The next great surge of American migration will likely be a surprise.

The post-1970 surges of migration, like surges of migration in the past, have led many to question whether and how Americans with diverse cultural, religious and political beliefs can live together. This is a question that Americans have always grappled with, not one posed by the sudden transformation of a long-homogeneous country into one with cultural and racial diversity.

The Framers of the Constitution and the Bill of Rights were well aware of the different religious and cultural backgrounds of the different states. They were determined to create a strong federal government but one whose powers would be limited in order to reduce cultural conflict and preserve zones of autonomy. This was a revolutionary approach to constitution-making, adopted when England still required public officials to be members of the established Church of England and when, in all European nations, Jews were subject to civic disabilities, including prohibitions on holding public office.

The Framers’ formula—limited government and individual rights—hasn’t always been applied faithfully in American history, and it wasn’t enough to prevent the outbreak of a civil war. But it has provided a ready and useful template for the accommodation of diverse peoples, even as the nation has been peopled by successive and culturally diverse surges of migration.

Mr. Barone is senior political analyst for the Washington Examiner and resident fellow at the American Enterprise Institute. This essay is adapted from his new book, “Shaping Our Nation: How Surges of Migration Transformed America and Its Politics,” to be published Oct. 1 by Crown Forum.