From the article 'A million Europeans enslaved':

An American historian says that more than a million Europeans were enslaved by North African slave traders between 1530 and 1780, a time of vigorous Mediterranean and Atlantic coastal piracy.

The number of white European slaves is only a fraction of the trade that brought 10 million to 12 million black African slaves to the Americas over a 400-year period, historian Robert Davis says, but his research shows the slave trade was more widespread than commonly assumed. The impact on Europe's white population was significant.

"One of the things that both the public and many scholars have tended to take as given is that slavery was always racial in nature - that only blacks have been slaves. But that is not true," said Mr. Davis, an Ohio State University professor...

In a new book, "Christian Slaves, Muslim Masters: White Slavery in the Mediterranean, the Barbary Coast, and Italy, 1500-1800," Mr. Davis calculates that between 1 million and 1.25 million Europeans were captured by pirates called "corsairs" and forced to work in North Africa during that period.

And, from this longer blurb:

...Davis said the vast scope of slavery in North Africa has been ignored and minimized, in large part because it is on no one's agenda to discuss what happened.

The enslavement of Europeans doesn't fit the general theme of European world conquest and colonialism that is central to scholarship on the early modern era, he said. Many of the countries that were victims of slavery, such as France and Spain, would later conquer and colonize the areas of North Africa where their citizens were once held as slaves. Maybe because of this history, Western scholars have thought of the Europeans primarily as "evil colonialists" and not as the victims they sometimes were, Davis said.

Who knew? Here I thought I was just an Evil Oppressor, and now it turns out there lies victimhood in my past. Of course, I'm not as much of a victim as an Oppressor, but at least I'm not 100% Oppressor.

But, seriously, it wasn't just those "corsairs" enslaving Europeans. Other Euros enslaved other Euros. For instance, the word "Slav" is the same as the word "slave." The Slavs were enslaved by the Germans, and their name became the name for slaves. And, people around the world have occasionally enslaved people outside their group. Even sometimes those inside their group. There are still slaves in some countries today.

There's another report on the book here.