Why was the World War One called “the war to end all wars” 1. Because of the unprecedented number of deaths
2. Because of the unprecedented number of nations involved
3. Because many were optimistic that fighting like that would never happen again
4. Because many devastated by the unprecedented destruction
5. All of the above

This is still somewhat of a contested issue, but in general most would agree that it was because "b. European Crusaders, attempting to win back Jerusalem, enslaved most of the Muslim world." It wasn't so much that they were enslaved, but that they were constantly under attack.