IT he UN emerged from the ashes of the Second World War as an idealistic body dedicated to preventing a recurrence of the death and destruction wreaked on an unprecedented scale across Europe, Africa and Asia.
In autumn 1944, as Joseph Stalin and Winston Churchill scribbled their agreements for the division of influence in the Balkans on scraps of paper, US President Franklin D Roosevelt set out his vision for a body that could save the world from future strife.
"We must... continue to be committed with our allies in a powerful world organisation which is ready and able to keep the peace - if necessary by force," he said.
According to his biographer, Conrad Black, the president was driven by a sense that the US had outgrown its isolationism but could not behave like the old Great Powers, carving up the world in their own interests. He sought instead a civilised world order without rule by sheer might in which the UN would have to operate coherently and legally.
During the war, representatives of Britain, the US, Russia, and at times China and France, met to develop ideas for a world body, and on June 26, 1945, the UN charter was signed by 50 countries. It gave to the UN, which began on October 24 that year, the task of safeguarding world peace, protecting human rights, establishing equal rights for all peoples, and improving the world's living standards.
But post-war rivalries and the conflicting interests of the countries that made up its membership meant that the UN was divided from the outset.
Churchill, for instance, saw the American support for self-determination everywhere not just as a drive against communism but as an attempt to prize Britain's fingers off its empire and the soft markets and cheap resources of its colonies.
The UN's precursor, the League of Nations, had been set up by the Allies of the First World War for the same humanitarian purposes, though without the US, but had failed to prevent the Second World War.
The UN could not secure its vision without the co-operation and widespread agreement of its members on key issues dealt with in its Security Council and General Assembly. But in many cases the descent of the Iron Curtain across Europe and the onset of the Cold War made such consensus an impossible dream.
As the "wind of change" swept through Africa and Asia in the 1960s, the success of the drive for self-determination increased memberships, and new voting blocs - including the non-aligned movement - challenged the interests of the powerful founder members. When the rivalry between the Western capitalist sphere and the Communist bloc receded as the Berlin Wall collapsed, a new world order emerged with the US as the sole superpower.
The release of the Soviet Union's grip on its satellite states left a vacuum that was filled by a succession of regional conflicts which forced the expansion of UN peacekeeping operations, quadrupling its original size, leading to some miserable failures.
Meanwhile, the work of diplomats became ever more intense. Before 1990, weeks often went by between Security Council meetings. Afterwards, large areas of foreign policy that were no-go areas in the Cold War became on-limits. The superpowers no longer vetoed UN intervention in their spheres of influence, and Third World countries no longer fought wars as superpower proxies. Lord Hannay, Britain's Ambassador to the UN (1990-95), recalls: "After 1990, the Security Council met not just five days a week but seven, and behind closed doors. The meetings were almost continuous."
The world body's zenith, reached with the broad coalition action against Saddam Hussein's invasion of Kuwait in 1990-1, has given way to a period of soul-searching after the divisions over the more recent war in Iraq and the US decision to go it alone with a little help from Britain.
Now, with 191 members - compared with the original 51 - the complexion of the UN has changed dramatically since its inception. And so have the challenges it faces. If a balance sheet of its achievements and failures in its first 60 years were drawn up, the successes would have to include the Security Council's record in helping to bring a peaceful resolution to some 172 conflicts, including the Iran-Iraq war, El Salvador's civil war and the Soviet occupation of Afghanistan.
UN peacekeeping operations have helped to cement ceasefires, deter renewed hostilities, enable free and fair elections, monitor troop withdrawals and bring political stability in many conflict countries, including Mozambique, Cyprus, El Salvador, East Timor, Cambodia and Macedonia. There have also been outstanding achievements in health. For instance, the UN's offshoot, the World Health Organisation, has eradicated smallpox worldwide and polio in the western hemisphere. It has also saved millions of lives through its immunisation programmes and community-based public health services.
Also on the plus-side is the work of the UN High Commissioner for Refugees.
Last year alone it helped some 17.1 million refugees and internally displaced people through its provision of basic aid, education and repatriation assistance.
But perhaps its greatest feat lies in the way the UN has served to uphold human rights. Its Universal Declaration of Human Rights of 1949 set a standard to which more and more countries have adhered over time. Its International Criminal Court, set up in 2002, prosecutes individuals for genocide, war crimes and crimes against humanity, and its 1989 Convention on the Rights of the Child is the most universally accepted and legally binding human rights document in the world, incorporating civil and political, social, economic and cultural rights. It has been ratified by every country except the US and Somalia.
Conversely, critics can quote a roll-call of dishonour: the failure of UN peacekeepers to restore order and protect food aid to Somalia; the 1995 massacre of 8,000 civilians in Srebrenica, in Bosnia, under the noses of insufficiently armed peacekeepers; the Rwandan genocide of 1994 in which 800,000 people were killed while UN peacekeepers could do nothing; and the Iraqi Oil for Food Programme, from which millions of dollars were allegedly siphoned by UN officials and Saddam Hussein.
Cambridge historian Andrew Roberts says the UN's record on peacekeeping and conflict resolution shows that the world body is a source of great complication and irritation. "When wars have been prevented, it has been thanks to bilateral discussions between the countries involved, not because of the UN," he says.
But one of its weaknesses may also be a great strength. The UN's democratic structure requires two-thirds consensus on major issues among 191 diverse member states in the General Assembly. Linda Fasulo, the UN correspondent for NBC News and author of An Insider's Guide to the UN, says: "The United Nations reflects the diversity of the world's governments. If Security Council members vote against something, it won't happen. If countries don't offer peacekeepers, the UN can't do anything to keep the peace. Considering the disparity of member states' political systems, it's almost a miracle that anything gets done."
Sir Brian Urquhart, one of the world body's founders and former under-secretary general for special political affairs, whose chief role was in peacekeeping operations, agrees. "Everyone lost credibility after Srebrenica and Rwanda," he says.
Yet the UN is no less credible than any of its member states, and while the UN's system of consensus is "an extremely inefficient process", people should be realistic, says Sir Brian.
"What people don't understand is that it's an experiment," he adds. "In its 60 years, it has had to deal with the Cold War, decolonisation, and now international terrorism and crime. It has to deal with these things without having national sovereignty or a constitutional arrangement. But it is the best organisation you'll get the world to agree with - it just has to get better."