There are three reasons often given to allow just about anyone into America. The first is that people should have the right to go where they please. The second is that multiculturalism and diversity is good for a country. The third is that even foreigners with widely divergent beliefs and attitudes will be converted by the American way of life and embrace freedom, openness, and tolerance.
Regarding the right of anyone to go where they please: Let's say there's a Marxist rebel army in Mexico. Are we saying that, if they are on the run with the Mexican army in pursuit, they have a right to retreat into America?
Can this rebel army cross back into Mexico for guerrilla raids, and then retreat back to their new base in the United States?
Should they be allowed to stay and establish their desired communist dictatorship within the United States itself?
Don't we have a right to keep out not only criminals, but also dissidents with anti-liberal views? Don't we have a right to keep out those who would overthrow our political system and our laws if they could?
On to multiculturalism. There is mounting evidence that the more diverse a neighborhood, the more crime and less community involvment – but let us leave that point aside. Let's instead imagine that a German neo-Nazi is fleeing Europe's hate-speech laws and seeks asylum in the U.S.
Is another neo-Nazi voice something we really need? We may want cultural diversity and differing viewpoints, but must we presume different viewpoints are equal to each other? Is so, why not let the neo-Nazi in? Are we to say he can broaden our perspective and enlighten us? While we tolerate intolerant opinions expressed by Americans exercising their free speech rights, how is it helpful to add to their number?
As to converting immigrants to the American way of life. Let's say members of an apocalyptic cult from Japan, India, England, or wherever seek to enter America with the intention of winning more converts before embarking on a campaign to get their leader, who they call the Messiah, crowned King. Would the United States have a moral obligation to welcome in such nut-jobs? Should we assume that, once here, they will see the benefits of liberal democracy and give up their cult?
Such an assumption would be an unfounded faith in the ability of liberal democracy to turn everyone into liberal democrats. All too often, what we see as prosperity and freedom, others see as decadence and blasphemy. Liberal democracy isn't a religion, and "faith" in its laws, systems, and culture won't save it from its enemies.
An English-speaking, highly-skilled immigrant with a clear criminal record may want to become a permanent resident of the United States. Just the same, if his beliefs are hostile to our social systems and our laws – if he would overturn them if he could - it makes little sense to admit him, or others with the same beliefs.
Which brings us to the problem of Islam. Islam is not just a religion, it is a political ideology. As Muslim immigration and birth rates increase their numbers, they are more likely to insist on enforcing their laws amongst themselves. Over time, their numbers could turn into majorities in certain neighborhoods, then cities. With increasing political power, there is an increased likelihood of Islamic law, or Islam-inspired law being enforced upon everyone.
If Islam is a threat to the United States and Western world, the threat comes not from terrorist attacks, because terrorism is not a means of conquest. Indeed, terrorism on U.S. soil is strategic folly, because it would probably create a backlash against Muslims already here and cause a crackdown on further Muslim immigration. It is more likely that if Islam prevails, it will through demographics, through population growth – first in Europe, then in North America.
If Western countries want to keep that from happening, they shouldn't let Muslims move into their lands.