Around the turn of the last century, Theodore Roosevelt offered some blunt but friendly counsel to the large numbers of immigrants then making their way to America. "We have no room," he declared in 1894, "for any people who do not act and vote simply as Americans, and as nothing else." He noted that large numbers of immigrants had become "completely Americanized," and they stood upon the same plane as "the descendants of any Puritan, Cavalier, or Knickerbocker among us." But, he added, "where immigrants, or the sons of immigrants, do not heartily and in good faith throw in their lot with us, but cling to the speech, the customs, the ways of life, and the habits of thought of the Old World which they have left, they hereby harm both themselves and us." Immigrants who remain "alien elements, unassimilated, and with interests separate from ours," said Roosevelt, become "mere obstructions" to the current of American life. He summed up, "We freely extend the hand of welcome…to every man, no matter what his creed and birthplace, who comes honestly intent on becoming a good United States citizen…, but we have a right, and it is our duty, to demand that he shall indeed become so, and shall not confuse the issues with which we are struggling by introducing among us Old-World quarrels and prejudices."

By today’s standards of political correctness, this TR manifesto, as it might be called, was an almost breathtaking demand for assimilation on the part of the country’s immigrants. It assumed a prevailing American culture to which newcomers would have to adapt if they wished to be successful in the New World. This view, prevalent and unremarkable in TR’s time, eventually was challenged by those who embraced what they called "cultural pluralism"—the idea that assimilation into a prevailing American culture was neither possible nor desirable and that America should encourage all to retain whatever cultural thoughts, impulses and sensibilities they brought from their lands of origin. That is the prevailing view today among intellectuals and policy experts.

These historical musings are prompted by the horrific human destruction wrought in Boston last week by Tamerlan and Dzhokhar Tsarnaev, whose stealthy bomb assaults certainly seemed motivated at least in part by Old World impulses and sensibilities. Though many politicians and commentators seem intent on brushing aside the fundamental questions of immigration and assimilation posed by those pitiless acts, they really can’t be ignored. We must ask ourselves how we invited into our midst children from another land who would grow up to wreak such havoc upon innocent Americans going about their daily lives in their own country.

Journalistic coverage has focused, and no doubt will continue to focus, on every aspect of the lives of these two young men of Chechen origin—their family histories, their experiences on the edge of a horrendous ethnic and religious conflict, their difficulties making it in America, their parents’ separation, Tamerlan’s religious odyssey, his psychological dominance of his younger brother. All this is relevant and warrants attention. But none of it is likely to answer in any satisfying way the question on everyone’s mind: Why did they do it? As the old 1930s radio show used to ask, "Who knows what evil lurks in the minds of men?"

But if we step back and look at it in a larger context, the questions take on a broader scope—and perhaps a more ominous tone. These two brothers proved themselves incapable of heeding the Roosevelt manifesto, of leaving behind Old World quarrels and prejudices, of somehow fitting their intensifying Muslim faith into the everyday customs and mores of a welcoming American society. As Tamerlan told a campus-publication interviewer before his brutal assault on innocents, "I don’t have a single American friend. I don’t understand them."

This raises a serious question about the level of success demonstrated by Muslims generally in assimilating into the Western societies that have received them over the past generation or so. The answer is that it is not a very high level of success at all. Of course hackles are inevitably raised among devotees of cultural pluralism whenever such broad generalizations are expressed. And these critics are correct in noting that the vast majority of Muslims blend into their adopted Western societies just fine. But large numbers of Islamic immigrants have had difficulty accepting the underlying precepts of their adopted Western nations, and currents of hostility run through the Muslim communities in those nations, including the United States.

"Muslims, particularly Arab Muslims," wrote the late Samuel P. Huntington in his last book, Who Are We?, "seem slow to assimilate compared to other post-1965 groups"—post-1965 groups being the large numbers of non-Westerners who migrated to the United States after the Immigration and Nationality Act of 1965, which ended the previous "national origins" formula that brought to U.S. shores primarily western and northern Europeans and replaced it with a preference based on skills and family relationships with U.S. citizens or residents. Huntington noted a Los Angeles poll showing that, as poll officials put it, "a significant number of Muslims, particularly immigrant Muslims, do not have close ties or loyalty to the United States." The poll revealed that 57 percent of immigrant Muslims and 32 percent of those born in America said that, if given a choice of staying in the United States or migrating to an Islamic country, they would opt for an Islamic country. Fifty-two percent of the interviewees said it was "very important" to replace U.S. public schools with Islamic schools, while another 24 percent said it was "quite important" to do so.

Huntington draws a distinction between national security, concerned primarily with sovereignty, and societal security, concerned largely with national identity, the ability of a people to maintain their culture, institutions and way of life—in other words, the very things Theodore Roosevelt was concerned about in issuing his 1894 manifesto. "In the contemporary world," writes Huntington, "the greatest threat to the societal security of nations comes from immigration." And immigration without assimilation constitutes the greatest threat of all.

That’s because it often leads to internal immigrant communities that are isolated and insulated in varying degrees from the broader society. This has happened, of course, with North Africans in France, Turks in Germany and other immigrant groups in other European countries. And in nearly all instances there has been a backlash against immigration in general. Writes Huntington: "Immigration without assimilation thus generates countervailing pressures and usually cannot be sustained indefinitely."

And yet there appears to be little recognition of this in the United States today, as reflected in the current congressional debate on immigration reform and in the journalistic reaction to the Tsarnaev bombings. In Congress, little focus has been devoted to the assimilation question posed by those bombings, and Judiciary Committee chairman Patrick Leahy of Vermont aggressively sought to intimidate his opponents in the immigration debate by alleging they were bent on using the Boston killings to "derail" the immigration bill. "A nation as strong as ours," he said, "can welcome the oppressed and persecuted without making compromise in our security." That sentence may be a bit hard to swallow by the families of those killed and maimed on Boylston Street.

And few journalists explored the story in the context of the assimilation question. Two exceptions bear notice. The Wall Street Journal, in its April 20-21 editorial, discussed the phenomenon of Muslim alienation turning into jihad in Western countries, citing particularly the London bombing of 2005, perpetrated by middle-class Pakistani immigrants from Birmingham, and the failed Times Square bomb attempt by Faisal Shahzad, a naturalized U.S. citizen from Pakistan. "After the London bombings," wrote the Journal, "many Americans took comfort in the belief that immigrants to the U.S. are better assimilated than they are in Europe. But that may be more conceit than fact, at least in regard to some young men."

And Anne Applebaum, writing in the Washington Post, suggested also that the Tsarnaev brothers resemble "the second-generation European Muslims who staged bombings in Madrid, London and other European cities." Educated and brought up in Europe, these young men nevertheless felt alienated there. Suggesting this phenomenon may be making its way to America, she writes, "We don’t expect to hear it from someone who grew up in Boston, a city that has taught generations of foreigners to become Americans in a country that likes to think of itself as a melting pot. But now it might be time to change our expectations. These terrorists are a lot less like the 9/11 attackers…and a lot more like the men known as the Tube bombers of London or the train bombers of Spain. Our response is going to have to be different—very different—as well."

Whether a different response is in prospect remains highly speculative in a country that has abandoned the TR manifesto in favor of the cultural plurality advocated by people such as Senator Leahy—who, in warning his opponents about inserting the Boylston Street bombings into the immigration debate, issued a plea for "the dreams and futures of millions of hardworking people," presumably future immigrants. It could be argued that Leahy and other national leaders have a higher responsibility to protecting the lives and limbs of current U.S. citizens enjoying the innocent experience of running or witnessing a marathon race in a great American city.