Mine's the tomato soup with Asian spice, easy on the Mexican bits

In 1935, Alistair Cooke applied for the new post of BBC representative in North America. He failed to get the job, in part, as he recalled in later years, because he speculated during the interview that his knowledge of the North American scene might be enhanced if he learnt Spanish.

Today few would question the relevance of the Spanish language to a full understanding of life in the US. Over the past 30 years, inner-city areas throughout the country, and a swath of territory from Texas to Southern California, have become essentially bilingual.

This change is at the heart of the new book by Harvard University professor Samuel Huntington, who famously charted the coming clash of civilisations between Islam and the West. But for Huntington this is not merely an issue of change in America, it is a challenge to America. The professor who built his career analysing the Soviet enemy and revived it by pointing to the Islamic threat is now warning about the menace posed by the poorest and most disadvantaged group in American society - Hispanics, specifically Mexican-Americans.

Huntington explains that this book grew from his twin observations of the surge of patriotism after the attacks of 9/11 and his sense of a crisis over the nature of US identity stemming from decades of multiculturalism previous to that event. Before launching into his headline-grabbing attack on Latinos, Huntington delivers a review of the academic literature on US identity and immigration that often betrays its origins in an undergraduate course. Huntington stresses two points particularly. First, America is not a nation of immigrants but a nation built by Anglo-Protestant settlers who later received immigrants who accepted their creed of God-fearing individualism and hard work. Second, US immigration has changed since 1965, with migrants now coming from a single linguistic base - Hispanic - and, he claims, displaying less inclination to assimilate or invest emotionally in their new country. His preferred image of US immigration is neither the "melting pot" nor the "salad bowl", but rather "tomato soup": an Anglo-Protestant concoction into which Jewish croutons, Catholic herbs and Asian spices have been sprinkled, enhancing the flavour without changing the essential identity of the dish. He sees Mexican-Americans as an ingredient that can only be added to the soup with time and care.

Huntington feels that the large-scale migration of people into the US with a different language and culture will compromise the time-honoured American brew. His worst-case scenario is the de facto Mexican re-conquest of Arizona, New Mexico and Southern California.

This is not the first time an American intellectual has rung the immigration alarm bell. Irish, Catholics, Germans, Jews and Asians were all demonised as culturally indigestible in their time. Huntington himself claims to be objectively presenting facts, but his language is deliberately inflammatory. Newcomers establish "beachheads" in the US, and Mexico is the source of "an illegal demographic invasion". Yet, behind the provocative language, genuinely interesting phenomena may be traced. Huntington reveals a world in which the wealthiest corporations and the poorest migrant workers alike embrace multiple identities and function with minimal attention to borders. He presents fascinating evidence of the way in which new arrivals in America are able to live as what he terms "ampersands", operating within the US while still rooted in their communities of origin.

He writes of entire towns that exist in both the home country and neighbourhood of New York, with citizens moving freely between employment in both, and American cities becoming essential campaign stops for Latin American elections. But he sees this phenomenon as negative: the ampersands are merely taking from America.

There are numerous flaws in Huntington's argument. He relies on crude statistics to lump together a diverse community that includes both poor and rich - migrant farm workers, the army general commanding US forces in Iraq and people whose residence predates the arrival of English-speaking Americans - with the most recent arrival to have chanced his or her life on the US-Mexican border. He jumps between Cuban, Salvadorian and Mexican-Americans with little regard for the distinctiveness of these groups. He is also much too quick to differentiate contemporary Mexican-American immigrants from previous generations of arrivals. While 21st-century telecommunications and cheap air travel have clearly made cross-border living possible as never before, Huntington gives too little attention to the border crossing of the past. Mexicans did not invent the idea of spending a limited time abroad and then returning, and the history of that predecessor of the ampersand - the "sojourner", who spent a portion of his or her life abroad and then returned - is little understood.

According to the Harvard Encyclopedia of American Ethnic Groups , between 1899 and 1924 the figures for departure against arrival from the US represented 50 per cent of Italian, 25 per cent of English (also the overall average), 12 per cent of Irish and even 5 per cent of Jewish immigrants. These people did not undermine the American project and seem to have disseminated American ideas at the grass roots on their return.

Huntington speaks of his patriotism and pride in America's past, but his tone reveals an undercurrent of uncertainty. This book is not the product of cultural confidence but of fear. One can equally respond to the coming of Spanish America with excitement at the new and vibrant cultural forms that are already being created. Similarly, the sojourner and the ampersand are not necessarily detriments to the host nation but could also be important resources for the international communication of its values: instruments of "public diplomacy", or what Huntington's Harvard colleague Joseph Nye has called "soft power". Returning sojourners will carry ideas about the US whether Americans like it or not. The US would do well to present the newcomers with a tolerant, open, confident society that is either worth joining or that has values worth translating back to their countries of origin.

Nicholas Cull is professor of American studies, Leicester University.