Prime Minister Blair is due in Washington later this week. He was, typically, the first Western leader to fly to Iraq to bolster the new government led by Prime Minister Maliki. When he arrives in Washington he’ll be welcomed not only by President Bush but an admiring Congress and the American people. The sad truth is that Mr. Blair is far more popular here than he is at home.

No other European leader would have taken the political risk of linking his own place in history to the American-led war on terror. That Europe is the weak link in the alliance against Islamist jihad is hardly news. We are used to reports of perfidy from Paris, betrayal from Berlin, mendacity from Madrid, squeamishness from Scandinavia. Americans think of the British, however, as loyal of allies in war and peace. We think of Winston Churchill, Margaret Thatcher and now Tony Blair as friends in foul weather as well as fair.

The thing to keep in mind this week is that Britain is under attack not only from without but from within. The British author and journalist Melanie Phillips, in a new book, offers a startling account of the transformation of London into “Londonistan,” the hub of the global Islamist terror network, notorious among intelligence agencies as a place where terrorists and those who inspire them could find refuge and disappear. According to Ms. Phillips, the London bombings in July disclosed the extent of Islamist penetration of British society, with extremist preachers in British mosques indoctrinating young Muslims, training them for jihad, and despatching them to attacks America, Israel and other targets. Several of the most infamous terrorists, including Zacarias Moussaoui, were recruited in London by such figures as Abu Qatada, Abu Hamza and Omar Bakri Mohammad.

What has made the unthinkable possible is the radicalization of Britain’s Muslim minority since the Rushdie affair in the late 1980s. Their increasingly aggressive demands have been accompanied by the emergence of multiculturalism, which leads to official appeasement of fundamentalism, an interpretation of human rights doctrine that can be exploited by the Islamists to undermine Western values, and a rise of anti-Semitism, which has regained respectability even within such institutions as the Church of England under the guise of “replacement theology.” As a result of this hollowing out of British identity, as it has been called, the victims rather than the perpetrators of Islamist terrorism are made into scapegoats: Israel and the U.S.

Why should this new battle of Britain matter to Americans? Partly because Britain is a land that nurtured many of the basic ideas that have made Western civilization possible: the rule of law, constitutional government, parliamentary democracy, religious toleration, freedom of speech and the press. Partly because moral and cultural relativism and victim culture are eating away at American society, too, and there are lessons to be learned from Londonistan. But most of all because the loss of America’s closest and most important ally would be a serious setback for the war on terror.

No doubt differences obtain between in America and Britain, but in both cases there is, in Ms. Phillips view, fertile ground for an alliance between Leftists and Islamists. In the run up to World War II this was called the Red-Brown alliance, after the communists and the fascist brownshirts. In any event, it is nothing less than a new battle of Britain, a battle of ideas that is every bit as important as the famous battle fought in the skies over Britain against Hitler’s Luftwaffe in 1940.