Specifically, the part where in the early 19th century America reconstituted the U.S. Marine Corps to battle the Islamic Barbary Pirates, though I don't think that's what Obama meant.
Here at home, we will strengthen our defenses but not succumb to a siege mentality that sacrifices the open society and liberties and values that we cherish as Americans, because great and proud nations don't hunker down and hide behind walls of suspicion and mistrust.