President Bush and others have said we are at war to save our civilization--that is, Western civilization. The events of Sept. 11 were perpetrated by Muslim radicals. All were foreigners; some were here illegally.

Understandably, many immigrants have been detained. Deportations of illegal immigrants have accelerated here and in Europe. There is speculation about an anti-immigrant backlash. The quadrennial candidate Pat Buchanan's book was high on bestseller lists. Its title says it all: The Death of the West: How Dying Populations and Immigrant Invasions Imperil Our Country and Civilization. If we're talking about the survival and, one hopes, the extension, of Western civilization, we should be talking about immigration and demographics. Let's get some numbers straight. Birth and fertility rates in Europe and Japan are unprecedented and incredible.

Over time, it takes 2.1 children per woman just to keep a society at zero growth, the "replacement level," absent immigration. The total fertility rate of Europe and Japan is 1.3 children per woman--on a 50-year slide. Europe has begun "de-populating"; Japan will follow soon, according to U.N. projections, which show Europe losing 124 million people by 2050, while its median age climbs to about 50.

Economic markets are responsive to change, but this decline could pose a challenge. How would you like to be a home builder in a society that is losing significant population every year? Where do the public funds for the military come from when there aren't even enough worker bees to provide pensions for the senior bees? These are not the best of all possible allies. America may have to move further toward a go-it-alone position.

Fortunately, the other part of the West, the English-speaking settler nations--Canada, Australia and the United States--have different demographic portraits. Why? Because, in moderate numbers, they take in immigrants. And the numbers are only moderate. In our peak immigration decade of 1900-1910, America accepted immigrants amounting to 1 percent of its population, each year. The rate today is one-third that.

America's fertility rate has averaged about two children per woman in recent years--below replacement. Over time, if it weren't for immigration, America would stop growing and perhaps begin slowly depopulating. Such a no-growth, aging America could find it hard to lead in what may be a long struggle among civilizations, some of which, unlike the West, are projected to grow for several decades before leveling off.

The flash point of American immigration today concerns Mexicans. To those, such as Buchanan, who fear immigration, they appear as an inexorable Third World tan tide, intent on recapturing the American Southwest, ready to swamp Western values, the splitting wedge in a majority nonwhite America of the future.

Bunk. America takes in about a million immigrants each year, of whom about 45 percent are from Latin America, of whom Mexicans account for somewhat less than half, perhaps 25 percent to 30 percent of the total when illegals are counted. The other 70 percent to 75 percent of immigrants to this country come from everywhere, including 30 percent from Asia.

Nonwhite? Half of Mexicans are classified white. Third World? Mexico's per capita income is above $5,000; it now has "investment grade" bond status. Mexico is a member of the Organization for Economic Cooperation and Development, the "First World" economic club. It is a real democracy. Its fertility rate has fallen from 6.5 to 2.5 children per woman in 30 years, and it is expected to go below replacement soon.

Invasion? How many invaders compete to mow the lawns and clean the dishes of those they have invaded? Mexican Americans serve disproportionately in this country's armed services and have a won a disproportionate share of medals for gallantry in combat.

Assimilation? By the third generation just about all Mexican Americans speak English, and Mexican grandparents often complain that their grandchildren speak no Spanish.

Not Western? Scholars say the padrones of Latin America, the hegemonic roots of the culture, are language and religion. The languages are Spanish or Portuguese, European tongues. The religion is Catholic, with growing evangelical Protestant sects.

Now is not the time to go wobbly on immigration. It is the key to American growth and prosperity. If we are indeed in for a war to preserve Western civilization, we'll need every straight-shooting, red-blooded, patriotic soul we can get. (Remember immigrants choose the American way of life.)

Immigrants are also our best salesmen; no one tells the American story better. And we're going to need all the salesmen we can get as we go about the business of recruiting economic, cultural and military allies--allies who believe in liberty, as we do.