In
March 1785, 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.[wiki]
Although Congress never voted on a formal declaration of war, they did
authorize the President to do what
had
to be done. And he did.
Because it's the President's duty.