GORDON Brown's new security adviser, Admiral Sir Alan West, warns that the terrorist threat will be with us for another 15 years. Does he think the threat will be over after 15 years? The lessons of history convince me that it won't.

Before 9/11 I was at a political conference with a group of academics discussing the Cold War. At the end the chairman said: "Well, at least the Cold War, the longest global conflict in modern times is over." At that point a wise old professor said quietly: "The Cold War was not the longest conflict. The longest conflict started 1,300 years ago - and it's still going on. In 1683 the Muslim armies were at the gates of Vienna and if we allow them, they'll be back again soon." As I said, this remark was made before the attacks on New York gave everyone perfect hindsight.

Islam has always organised its missionary work at the point of the sword. Many times Europe has come close to being conquered. For centuries there was Muslim occupation of much of Spain and Portugal and it was only the valour of the Frankish knights under Charles Martel in defeating them at Tours in AD732 which saved northern Europe falling into Muslim hands. In the Middle Ages, the Crusades rumbled on intermittently for 400 years. Again, in 1571, the warships commanded by Don John of Austria decisively defeated the Muslim fleet at the battle of Lepanto.

So throughout Europe for more than 1,000 years the usual scenario has been an on-and-off war with militant Islam. There have been centuries of peaceful co-existence but these have always been followed by periods of Muslim resurgence. The latest Muslim insurgence may be said to have begun with the downfall of the Shah of Iran in 1979 and the return to rule in that country of the fundamentalist Shi'ite Muslim Ayatollah Khomeini. I doubt whether Europe has the will to resist this latest Islamic aggression. Marcello Pera, a professor of philosophy and President of the Italian Senate, wrote recently about our policy of appeasement:

"A foul wind is blowing through Europe. This same wind blew through Munich in 1938. While the wind might sound like a sigh of relief, it is really a shortness of breath. It could turn out to be the death rattle of a continent that no longer understands what principles to believe."

Pera is a prominent academic and a statesman. He is the farthest thing you can get from being a fanatic. But he adds this warning: "Is there a war perhaps? Yes, there is a war and I believe that the responsible thing to do is to recognise it and say so, regardless of whether the politically-correct thing to do is to keep our mouths shut.

"In Afghanistan, Kashmir, Chechnya, Dagestan, Ossetia, the Phillipines, Saudi Arabia, the Sudan, Kosovo, the Palestinian Territories, Egypt etc large groups of fundamentalists have declared a Holy War on the West. This is not my imagination. It is a message they have proclaimed, written, communicated, preached and circulated in black and white. Why should I not notice it?"

Because, Professor Pera, we prefer to keep our heads in the sand and hope that when we allow ourselves to peep, our enemies will have disappeared. But they won't...