Americans are at last waking up to the threat posed by British-born Islamists.Since the exposure of a plot to blow up transatlantic airliners three weeks ago, alarm bells have been ringing in publications as diverse as the New Republic and the Wall Street Journal. The Investor’s Business Daily has even demanded an end to the visa waiver program, which allows four million Britons to enter America every year without visas. The Heritage Foundation’s Nile Gardiner describes Britain as “a hornet’s nest of Islamic extremists” and warns that Congress will react harshly to terrorist attacks emanating from Britain.

The fact that Americans are worried is gratifying. This column was among the first to warn about the radicalization of the British Muslim community. But there is a risk that, having ignored the danger hitherto, Americans may now overreact by penalizing all Britons, not just the minority who really do threaten security.

It is true that opinion polls show that a significant proportion of British Muslims have at least some sympathy for jihadist extremism, and that even their leaders are unwise or unscrupulous enough to use the threat of terrorism to put pressure on Tony Blair to abandon his support for America and Israel.

It is also true, however, that the overwhelming majority of non-Muslim Britons are just as hostile to Islamist terrorism as Americans. Recent polls have reinforced the impression that the British no longer need persuading — as they still did after 9/11 or during the invasions of Afghanistan and Iraq — that the West is engaged in a war on terror that may last for many years and will demand great sacrifices if it is to be won.

Even more significantly, the British — like the Americans — are now much more concerned about Islam than was the case five years ago. They no longer believe the assurances of “moderate” Muslim leaders or their non-Muslim apologists that Islam is a religion of peace. People are much better informed and understand that there is a real problem about Islamic theology, which is constantly used to justify jihad against America, Britain and Israel, while suicide “martyrs” are glorified.

About half of all Britons now see Islam as such, not merely its most extreme versions, as a potential threat to their way of life — not before time. It is not only the war on terror that has to be won; there is a culture war, too. This involves resisting the encroachments of aggressive multiculturalism, which acts as a Trojan horse for Muslim demands to live under Shariah law or to censor legitimate criticism or comment.

One deep-seated problem arises from the doctrine that all humanity is born Muslim. This implies not only proselytizing among non-Muslims, but laying claim to those who have yet to convert to Islam.

I heard this week about a young Asian woman whose father was a non-practicing Muslim. She and her siblings had been brought up by their mother as Christians, with their father’s consent. When she arrived at medical school in the northeastern city of Hull, however, she immediately found herself visited by fellow students who sought to win her over to their Islamist beliefs. They disapproved of her sharing a house with Hindu or Christian friends, and treated her as if she were a Muslim. I have also been told by a non-Muslim student of demands by her Muslim peer group to cover her head.

Such demands can and should be resisted, but in other circumstances they are backed by violence. The Fox TV men who were released by Hamas terrorists in Gaza earlier this week had been forced to convert to Islam. I have yet to hear this practice condemned by Muslim clerics, who appear to regard conversion by fair means and foul as legitimate. Conversion to Islam is also irreversible — the punishment for apostasy is death.

It is worth recalling the last time that Western civilization faced a comparable threat from Islam. From the 15th to the 17th centuries, Europe lived in permanent fear of the Ottoman Turks, whose relentless jihad conquered first the Byzantine empire and much of the Mediterranean, then the Balkans, and was twice halted only at the gates of Vienna.

During this era, voluntary conversion to Islam was seen as “turning Turk,” and treated in Christian Europe as potential treason. But forced conversion was also practiced by the Turks, and on a massive scale. Indeed, the elite of the Turkish army — the Janissaries — consisted of such converts or their children.

Today, a common sight on the streets of London and other British cities is a stall with Muslim missionaries advertising their faith. Everywhere you see “Islamic bookshops” which sell not only the Koran, usually described as “the last testament,” but also poisonous anti-Semitic and anti-Christian propaganda. That is unacceptable.

In a free society, all religions enjoy the right not only to practice but also to pass on their faith to the next generation and to make new converts. I wonder, however, if these rights are being abused in Britain today. There have been a number of horrible “honor killings” of Muslim women by their fathers or brothers, as punishment for marrying or even associating with non-Muslims. These barbaric murders are rarely condemned in public by Islamic clerics.

If Islam is ever to find a modus vivendi with the rest of British society, it is going to have to change at least some of its doctrines. The ambiguity about jihad, one of whose Koranic interpretations is holy war, must be ended. I am not comfortable even with the “peaceful” interpretation of jihad — the talk of “inner struggle” sounds too much like “Mein Kampf” for my taste. But there can be no toleration of a faith that excuses terrorism under the rubric of “martyrdom.”

Nor is there any place in civilized society for forced conversions, including the denial of liberty to reject a family faith, let alone brutal punishments for apostasy. These doctrines must be unequivocally condemned if the suspicion that surrounds Islam in the West is ever to be dispelled. This is a matter of theology, not merely politics. I have yet to hear any such theological condemnation from Islamic scholars.

It is true that we do not face a threat as visible and direct as the Ottoman Empire once posed. But Iran’s defiant pursuit of nuclear weapons is potentially more deadly than the Turkish fleets and armies that were defeated at Lepanto and Vienna. Iran, however, would be much less dangerous without terrorist “sleepers” in the West.

The British are confused about how to win this war on terror. Many of them imagine that there is a “European” alternative to the Atlantic alliance, which would somehow make Britain less vulnerable to attack. There is no such alternative. The British can and must deal with their own internal Islamist threat, but they need their American allies to deal with the external one.