Donald Trump is a lunatic. But the West really does have a homegrown terror problem.

When Donald Trump called for ending Muslim entry into the United States, the negative reaction from the rest of the Republican presidential contenders was swift and nearly unanimous. Perhaps the most emphatic came from low-polling Sen. Lindsey Graham, who called Trump a "race-baiting, xenophobic, religious bigot."

Graham has actually made similar comments before, including last week while narrating the bizarre spectacle of a reporter sifting through the San Bernardino shooters' apartment on live television. Then, Graham suggested that Trump's rhetoric about Islam risked repelling the Muslim allies we need in the fight against ISIS. But predictably, the South Carolinian also said the most important thing in combatting terrorism is defeating caliphates and would-be caliphates on the battlefield overseas.

Is that really true? Yes, the Islamist militants in Iraq and Syria provide an example that inspires fellow travelers in the United States and Europe. Sometimes they also supply training, funding, and other support that cannot be ignored. But the recent attacks in Western democracies are primarily carried out by radicals in our countries, not abroad.

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Paris attack suspect Salah Abdeslam was born in Belgium. San Bernardino gunman Syed Farook was born in Chicago. Farook's wife and accomplice Tashfeen Malik was a permanent resident vetted by the U.S. immigration bureaucracy. The Tsarnaev brothers behind the Boston marathon bombing immigrated to the United States. Dzhokhar seemed as assimilated as they come.

None of these terrorists were ISIS infiltrators or plants on arrival. They subsequently became radicalized and turned against the countries in which they lived.

Don't let Trump's downright racist bluster obscure this fact: The West has a legitimate problem with assimilation among Muslim immigrants and their descendants. That problem is not as great in the United States, where the numbers are proportionately smaller and the national track record of assimilating immigrants is better, as it is in Europe. And it is far from universal.

Nevertheless, assimilation isn't automatic. Integrating people into vastly different cultures is hard work. And while this process often makes migrants burst with pride in their new countries with the zeal of a convert, it can also be a profoundly traumatic experience.

Alienated young men and women at most a generation removed from countries where high percentages of their coreligionists hold illiberal values can be susceptible to radicalization. While that radicalization will lead to violence in only a few cases, it doesn't take many to produce a San Bernardino shooting or Boston marathon bombing.

Addressing the immigration component of this issue, the aspect that is most easily within our control, while redoubling efforts to absorb and acculturate those who have arrived in recent decades will not solve everything. But it will almost certainly be a more effective contribution to the fight against ISIS than trying to resolve the Syrian civil war.

What Trump is calling for goes vastly too far beyond these necessary steps. His advocacy of a blanket ban on Muslim immigration and tourism, coupled with ambiguous statements about the status of American Muslims (in an interview with Fox News, he spoke as if allowing them to stay in their country was a magnanimous act) undercuts the idea that assimilation is achievable. It indiscriminately treats ISIS sympathizers and Muslims fighting ISIS the same.

Assimilation is a two-way street. You can't expect immigrants to become members of a community that will never fully accept them. And as the French have seen, you cannot demand Muslim respect for religious liberty and pluralism while not extending the same to Muslims.

As is so often the case with Trump, instead of pushing back against well-intentioned but naïve political correctness with nuanced arguments about how time, numbers, and cultural compatibility are of the essence with immigration and assimilation, he goes careening in the direction of the opposite extreme.

Restrictionists who should know better hope Trump is pushing the Overton window on immigration in their direction. He is instead reducing it to the stuff of dining room arguments about Starbucks' red holiday cups and sitcom squabbles between Archie Bunker and Meathead.

Mainstream politicians have also avoided tackling this sensitive topic and in so doing have ceded it to demagogues and rabble-rousers. Graham is right to say Trump's comments are unhelpful but wrong in his own diagnosis.

It is yet another reason, as The Atlantic's David Frum has written, that "the West appears headed to a bigger war in Syria, in response to killers who came from Belgium." And Chicago.