"We honor them with our presence and certainly with this memorial," Adm. Mike Mullen said at a Pentagon ceremony for those who died in the building and on the plane that hit it.
"Mostly, we honor them with our lives, with what we have done from that day to this - the sacrifices we have borne, the laughter we have shared, the hope we have dared to let back into our hearts."
The ceremony included family members of the 184 who were killed in the 2001 attack on the sprawling Defense Department headquarters.
The war to go after al-Qaida - and Taliban leaders harboring them in Afghanistan - is now almost nine years old.
"Today we also reflect on what those attacks meant for an entire generation of young Americans who answered the call to serve," Defense Secretary Robert Gates said of the armed forces.