With President Obama considering troop withdrawal, look back at the war in Afghanistan -- 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.
With Earth Day coming up, find out which 25 cities are considered the…
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