"We couldn't imagine that something like that could have occurred," he said, referring to the 9-11 attacks. "This is the test we need to hold ourselves to in the future: Try to imagine threats, expect the unexpected, and then prepare ourselves to deal with it."
He added that U.S. security officials "need to think like the enemy" in order to discern possible threats.
"Most of the big events in terms our security have been unexpected, the fall of the Berlin Wall, the Iraq invasion of Kuwait, the 9-11 attacks and the Arab Spring," Lieberman said. "We did not anticipate these events."
Lieberman noted that last Sunday was the 10th anniversary of the creation of the Department of Homeland Security, established on Nov. 25, 2002 by President George W. Bush and Congress.
In his "plus" column in assessing homeland security:
— Al Qaida has been unable to carry out an attack resembling 9-11.
— In the aftermath of hurricane Katrina, changes were made to the Federal Emergency Management Agency (FEMA) to improve its response to natural disaster.
But, he cautioned, homeland security "has no destination point" — meaning that it requires constant vigilance.
Noting that he was speaking on the George Washington University campus, Lieberman said he thought then-President Washington "was in my subconscious when I decided not to run again because he knew when to quit," a reference to Washington's decision in 1797 not to seek another term as president but, instead, to retire to his Virginia plantation.
Lieberman hasn't announced his future plans.