Dahler reported on the Columbine shootings in his native Colorado. On September 11, 2001, Dahler was the first network correspondent on the scene of the attack on the World Trade Center, reporting live via telephone from his apartment just blocks away a few moments after the first plane hit the towers and later filing the first report of the South Tower's collapse mere seconds after it occurred. Two weeks later, he was one of the first American journalists travelling to Afghanistan before the U.S. began its bombing campaign against the Taliban. In 2002, he became one of the first journalists from this country, along with a team from Nightline, to successfully cross the Syrian border into northern Iraq prior to the Iraq War in more than a decade.