The borders between Christendom and the Islamic world have shifted for over a millennium, and at several key moments has erupted into war.
The list of combatants from the past includes: Richard the Lionheart, Saladin, Suleiman the Magnificent, the Mahdi, Gordon of Khartoum and now George W Bush and Osama bin Laden.
The Crescent and the Cross, a four-part series, presented by Owen Bennett-Jones, examines several turning points in the relationship between Christianity and Islam covering Muslim Spain, the Crusades, the Ottoman Empire and the struggle for Africa.
It has recently become fashionable to argue that the gulf between Islam and Christianity is deep and eternal. The series assesses whether such a claim can survive the scrutiny of history?
Part One starts by look going back over 1,000 years ago, in what we now call Spain, but was then known as al-Andalus.
In June 2009 President Obama went to Cairo University to make a major speech aimed at reaching across the religious divide. Officials said that hundreds of people had been involved in drafting it. But for all the preparations the President made this horrible historical howler about Al Andalus or Muslim ruled Spain.
"Islam has a proud tradition of tolerance. We see it in the history of Andalusia and Cordoba during the inquisition."
Barak Obama had conflated – an era of relatively tolerant Muslim rule with the Christian intolerance of the Inquisition 500 years later. It shows how difficult it is to talk or write about the history of Islam and Christianity without making errors.
Soon after the death of Mohammed in 632, the Muslims rapidly conquered significant amounts of territory. And having swept through North Africa they first crossed into Spain in 711. "They were imperialists. One of the big differences between the development trajectory of Christianity and Islam is that Islam begins as a religion of empire. The prophet Mohammed himself is a military leader he's a man who creates a state, he has a vision of extending that state outwards."
According to renowned historian Professor Eduardo Manzano Moreno, the speed of the Islamic advance "was made possible by several things. Firstly they had a very solid military organization. Secondly they had a very cohesive message, which was Islam. Thirdly many times they had arrangements and pacts with local populations."
The Muslim armies did not stop at the Pyrenees. Indeed in 732 they reached within 300 kilometres of Paris before being blocked by Christian forces at the Battle of Poitiers. Even if there is little record of Poitiers in Muslim histories, for the Christians it was a decisive moment. The English historian Edward Gibbon later wondered what might have happened had the result of Poitiers been different.
"The Arabian fleet might have sailed without a naval combat into the mouth of the Thames. The interpretation of the Koran would now be taught in the schools of Oxford and her pulpits might demonstrate to a circumcised people the sanctity of the truth of the revelation of Mahomet."
Even if they never reached Paris, the Muslims were to rule Spain for several centuries. Córdoba in particular became a beacon of learning and one of the most advanced cities on earth. "I don't think that for the 10 century you have anything like this in the whole of Europe", says Eduardo Manzano Moreno. It had a vast library, running water in the homes, street lamps and elaborate palaces where the rulers or caliphs held court, One 10th century visitor described it as the "ornament of the world." The biggest building in the city was the mosque which could accommodate 17,000 people praying indoors.
Freedom to practice
The mosque has subsequently become a symbol of religious division. When the Muslims were eventually forced out of Spain, the Catholics took over the mosque, knocked a hole through the roof and built a cathedral in the middle of it. To this day the Catholic authorities insist it is a cathedral and not a mosque. Occasionally Muslims try to pray in the remaining Islamic parts of the structure but they are thrown out by guards who are constantly on patrol.
Of course in many parts of the Middle East churches have been turned into mosques and Christians barred from worshipping in them. There was a time when Cordoba was known for its religious tolerance. For long periods under Muslim rule, Christians and Jews were free to practice their own faiths.
By today's standards the Caliphs imposed some very strict distinctions between the different religious groups, Christians and Jews for example were only allowed to ride donkeys not horses. But by the standards of the time the three religions coexisted remarkably well.
Tourists visiting Cordoba today are so keen to learn about the period of tolerance, they tend to ignore less attractive aspects of the city's history. "Most of the people who come to Cordoba have a very romantic view," says city guide Isabelle Richter. "They all think that this was the place that all three monotheistic religions lived together peacefully, in harmony, and of course this happened but what hardly anybody knows is that a very radical force took power, the Berbs, and they persecuted the Christians, the Jews and liberal Arabs."
Muslim Spain is no exception to the rule that propagandists on both sides can cherry pick the parts of history that validates their view of the present and of the future too. While President Obama has tried to evoke Cordoba's relatively harmonious past, Osama bin Laden has called for Spain too be returned to Muslim rule.
The Crescent and the Cross is written and presented by Owen Bennett-Jones.
First broadcast on 09 November 2009.