"I would have loved to have done more, but we did a lot," Cohen told reporters, citing the cruise missile attack on bin Laden's positions in Afghanistan in 1998, and the attack on a pharmaceutical plant in the Sudan.
Cohen said the attack in Sudan shut down bin Laden's attempt to produce deadly VX nerve gas.
The former defense secretary said superior intelligence; superb cooperation with allies, and constant vigilance at home will help win the war.
"We may never know when he have won a victory," Cohen said. "We can eliminate state-sponsored terror, but there will always be independent groups."
"There are people out there who are trying to kill us, and will stop at nothing," he said. "They will wait and watch until our guard is down, and then will strike again."
Cohen warned in a speech to the American Society for Industrial Security that the United States must not neglect "conventional threats" while it focuses on the terror war, saying Iraq, Iran, and North Korea remain a danger to the United States.
Cohen said the attacks of Sept. 11 have made possible a wide-ranging coalition against terror, including the Russians, which did not exist during his term as defense secretary.
"They just saw it as an attack on our ship, the bombing of our barracks," he said. "Now the Russians and others see terror as more than a U.S. problem, but as a world problem."
Cohen warned that the entire U.S. defense mindset would have to adapt to a new threat that cannot be contained by conventional means.
"Normal notions of deterrent don't exist," he said. "If somebody fires an ICBM into the United States, that missile leaves a trail, and the person who fired it will be liquidated in about 29 minutes. But today we're dealing with exotic techniques, and we need higher levels of human intelligence, and higher levels of preparation, to deal with this threat."