Former U.S. President George W. Bush ordered the start of an invasion on Iraq on March 19, 2003. The so-called shock and awe campaign on Baghdad began early the next morning 10 years ago.
Bush's decision was as a response to an alleged threat of weapons of mass destruction from Iraqi leader Saddam Hussein. No such weapons were found.
Hagel commemorated the 4,475 U.S. service members who died during the war. U.S. combat forces left Iraq in December 2011 and it's time for the country to chart its own path, he said.
"The Iraqi people will determine the future of Iraq and the United States will continue to support their efforts for a peaceful, secure, free, and prosperous nation," he said in a statement.
A series of reports published as the anniversary approached were critical of the war. Hagel, when he served as a U.S. senator from Nebraska, was highly critical of Bush's foreign policy and the war strategy.
The Iraqi government said Tuesday it was delaying elections in Anbar and Ninewa provinces for six months because of a declining security situation, the Voices of Iraq news agency reports. Elections were scheduled for mid-April.
The Iraqi news agency reported that "hundreds of casualties" resulted from a series of explosions in Shiite neighborhoods Tuesday in Baghdad.