Strolling on a summer evening in a North African resort town, the vacationing Senegalese businessman could have forgotten he was anything but a Muslim among Muslims, an African among Africans.

But a shouted insult from an Arab policeman set the black man straight: "Son of a slave."

Along ancient Saharan trade routes, 1,300 years of shared history that have mingled the faiths, cultures and skin tones of Arabs and Africans has left another, more vicious legacy: Arab-African slavery (search) that has endured as long as the two peoples have been together, leaving black Africans fighting perceptions of themselves as lesser beings, and of Arabs as the civilizing, conquering force.

Today, the old roles are playing out at their most extreme in Sudan's Darfur region (search), with murderous results: Arab horsemen clutching AK-47s raze non-Arab African villages and drive off and kill the villagers, in what rights groups call an ethnic cleansing campaign backed by Sudan's Arab-led government.

To Pape Thierno Ndiaye, the Senegalese businessman who spent the mid-1990s in Arab-dominated North Africa, the message was simply that he was a lesser being than Arabs, and unwelcome among them.

"It was like that all the time," Ndiaye, now back home in Senegal, says of his time on the Arab-dominated northern edge of the Sahara, and of the policeman's insult in the Morocco beach town of Agadir.

"It was insults all the time — all of a sudden, the problem of color had become an ordeal," Ndiaye said.

In Sudan, experts say similar racism is the spark setting fire to Darfur. Up to 80,000 black Africa villagers are believed to have died, many slain by Arab Janjaweed (search) nomads competing with them for a fertile zone shrinking under desertification, and by a minority Arab government accustomed to keeping power by killing opponents.

With more than a million displaced, U.S. officials project a third of a million of Darfur's non-Arab Africans will die by the end of the year.

"You, the black women, we will exterminate you," Amnesty International (search) quoted one 20-year-old black African woman as telling them, speaking of the Janjaweed who abducted the women of her village in September 2003 and raped them for days.

With power and land at issue, Sudan's central government "is stoking racial and ethnic animus more than it ever has been in Darfur history," said Eric Reeves, a professor at Smith College in Massachusetts, and one of the leading academic experts on Sudan.

"It's the animating feature of the war ... on African tribal groups," Reeves said.

In southern Sudan, the common word for non-Arab Africans today among the Arab elite remains "abid," or slave. The general word for non-Arab Africans in Darfur, in western Sudan, is "zurga." The word "means closer to 'nigger' than 'colored,'" Reeves said.

It's important to note that only in Sudan are the African-Arab differences spilling out in blood.

Across the Sahara and its edges, the Sahel, most of the coexistence is peaceful, linked by shared cultures and by Islam.

Arabs and most Western academics agree that the Arab form of African slavery, existing since at least the 6th century Arab conquest, generally has been less brutal and more open to advancement by slaves than the Western version.

African slaves in Arab households largely have labored as servants in households rather than farmhands on plantations, making for easier lives and less submerging of identity. They often ate and slept side by side with their masters, sometimes married into their families — and occasionally even came to rule a Muslim kingdom.

But slavery ended in the West more than a century ago. It persists in the Arab world, in the West African nation of Mauritania (search) and in Sudan, human rights groups and Western governments say.

Arab-African differences have boiled up in blood in recent years in places other than Sudan.

In the 1990s, Mauritania's current leader oversaw a bloody purge of black Africans from the Arab-dominated nation's military.

In Mauritania today, some Arab officials routinely refer to even educated black African professionals as "slave people."

Sudan long has been one of the anchors of the Arab-African slave trade. Its appetite for slaves remains such that rebels in neighboring Uganda, a group calling itself the Lord's Resistance Army is alleged to trade African children to the Sudanese for an automatic weapon each.

Ironically, in Darfur and elsewhere, intermarriage between Arab and non-Arab Africans over the centuries has become so common that physical differences have ebbed or disappeared. The skin of the Arab Janjaweed militiamen is as dark as the African villagers they hunt down.

"They would say these are not real Muslims — these are pretend Muslims," said Richard Cornwall, at the South Africa-based Institute for Strategic Studies.

"Many generations of intermarriage have ensured there's not really a physiological difference," Reeves said. Often, however, the Janjaweed "clings to the notion of Arab racial identity. It's racism where there is no racial difference."