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.

America was peopled in very large part by surges of migration, immigrant and internal, which lasted only one or two generations and whose beginning and endings were mostly unpredicted. Some of these movements were prompted by economic incentives. But when large numbers of people uproot themselves, there is almost always something else at work. They are migrating to pursue dreams or to escape nightmares, to build new communities on which they can put their stamp.

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.