A new security agreement keeping U.S. troops in Afghanistan past this year marks yet another chapter in the longest conflict in U.S. history.
Sept. 11, 2001 -- Al-Qaeda operatives hijack four commercial airliners, flying two into New York's World Trade Center and a third into the Pentagon. A fourth plane crashes in a field near the town of Shanksville, Pa., after passengers storm the cockpit. Nearly 3,000 people die in the attacks.
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