Let's just add one more historical note to the matter...
In March 1786, Thomas Jefferson and John Adams went to London to negotiate with Tripoli's envoy, ambassador Sidi Haji Abdrahaman (or Sidi Haji Abdul Rahman Adja). When they enquired "concerning the ground of the pretensions to make war upon nations who had done them no injury", the ambassador replied:
"It was written in their Koran, that all nations which had not acknowledged the Prophet were sinners, whom it was the right and duty of the faithful to plunder and enslave; and that every mussulman who was slain in this warfare was sure to go to paradise. He said, also, that the man who was the first to board a vessel had one slave over and above his share, and that when they sprang to the deck of an enemy's ship, every sailor held a dagger in each hand and a third in his mouth; which usually struck such terror into the foe that they cried out for quarter at once."
Q.E.D.