Speaking in Kabul Thursday, Ms. Rice said the United States made a mistake by not staying engaged in Afghanistan after helping mujahedin fighters push Soviet troops from their country in the 1980s.
She said this lack of engagement led to Afghanistan's disintegration into a chaotic war, creating an environment that allowed the al Qaida terror network to grow and then make its attacks on the United States in 2001.
"After the Soviet Union left, I think it is well understood that we did not remain committed. And I said to the president [Karzai] earlier that in many ways, September 11 was a joint tragedy of the Afghan and the American people out of that period," she said.
The United States maintains about 18,000 troops in Afghanistan to help provide security, and is the country's largest foreign aid donor.
Ms. Rice said this time Washington will stay committed to Afghanistan's development.
"We will stand by the Afghan people as they go through the next phase of their democratic development, the parliamentary elections that will take place this fall," she said.
In September, the country plans to hold its first parliamentary election since Afghan and U.S. forces overthrew the former hard-line Taleban government in 2001 because it was sheltering al Qaida leaders.
The vote was originally scheduled to be held during last October's presidential election, but has been delayed because of security and logistical concerns.
Speaking alongside Ms. Rice, Afghan President Hamid Karzai said he favors the Afghan election commission's tentative date of September for the vote.
President Karzai also reiterated his administration's determination to stamp out illicit opium production in the country, though he said this would take time to accomplish.
"Nobody says nor will the international community say that we will be ending drugs in Afghanistan this year or even next year," said Mr. Karzai. "But we have begun a journey, and that journey has begun with a very positive note."
Afghanistan is the world's largest producer of opium, which is used to make the addictive narcotic heroin.
Ms. Rice's one-day visit to Afghanistan is part of a tour of Asia, with stops in India, Pakistan, China, Japan and South Korea.