United Nations ? Kofi Annan steps down as secretary-general at midnight today, leaving behind a global organization far more aggressively engaged in peacekeeping and fighting poverty – but struggling to restore its tarnished reputation.

Taking office six years after the collapse of the Soviet Union, Annan helped preside over a decade that saw the world unite against terrorism after the Sept. 11, 2001, attacks, then divide deeply over the U.S.-led war against Iraq which toppled Saddam Hussein.

At a Millennium Summit in September 2000, he spurred world leaders to adopt a blueprint to wage a global war on poverty and bring the United Nations into the 21st century.

Five years later, he called a follow-up summit to mark the U.N.’s 60th anniversary. Hoping to complete the bold changes, he sought to promote development, ensure international security and end human rights abuses. History’s largest gathering of world leaders took a first step, but it fell far short.

Unlike the upbeat atmosphere at the dawn of the new millennium, the World Summit in 2005 took place after a year of almost daily attacks on the United Nations over allegations of corruption in the U.N. oil-for-food program in Iraq, bribery by U.N. purchasing officials and widespread sex abuse by U.N. peacekeepers.

Kofi Annan, right, secretary-general of the U.N., shakes the hand of Ban Ki-moon, his successor, in this Oct. 13 file photo. Annan steps down as secretary-general at midnight today.

World leaders agreed to create an internal ethics office but they did not give Annan the authority to make sweeping management changes. The major overhaul of the U.N.’s outdated management practices and operating procedures will be left to Annan’s successor, Ban Ki-moon, who takes over Monday.

At a farewell news conference earlier this month, Annan said he considered his top achievements the promotion of human rights, fighting to close the gap between extreme poverty and immense wealth, and the U.N.’s campaign to fight AIDS and other infectious diseases.

“His greatest accomplishment was to set a framework that moved the U.N. from one century to the next – the response to mass atrocities, the central role of democracy, the importance of human rights, and a priority to development,” said Lee Feinstein, a fellow at the Council on Foreign Relations.