More than 100 world leaders are gathering in New York for the annual debate at the UN General Assembly.
The shadow of Iraq is again falling over the UN's annual session
The General Assembly is expected to address a wide range of issues, including the crisis in Darfur, Iraq and conditions in the developing world.
Over the past year, the members of the UN have been trying to overcome the deep divisions left by the Iraq war.
Iraq remains a key issue for the UN, which found itself largely sidelined after the fall of Saddam Hussein.
The UN Secretary-General, Kofi Annan, is expected to speak out at the meeting of the 191 member countries against those who disregard laws protecting civilians - citing examples in Darfur, Iraq and the Middle East.
A senior official in his office said the secretary-general would look at the idea that there "should be fixed rules of human conduct everyone understands that they should obey".
Problems in Iraq
The official said the secretary-general would not be discussing the legality of the military action in Iraq.
Last week, in a BBC interview, Mr Annan said the invasion of Iraq was illegal because it did not have Security Council approval, which sparked an angry reaction from the US and its allies.
However Iraq is still likely to feature in the debate.
"What is important to the US right now is to try to increase participation in solving the problems of Iraq," said the US Ambassador to the UN, John Danforth.
"I think it is a major issue before the General Assembly and before the world community in general."
US President George W Bush has said he will talk about the possibilities of improving healthcare, expanding prosperity and extending freedom as an alternative to terrorism and violence.
Another main issue is likely to be the crisis in the western Sudanese region of Darfur where tens of thousands of people have died.
A resolution passed by the 15-member UN Security Council on Saturday called for Sudan to rein in the militias accused of widespread killing, or face the threat of sanctions.
There could be pleas during the debate for the international community to provide more funding to help the humanitarian effort.
"Darfur is one of the burning issues of the day," said the current President of the General Assembly, Jean Ping.
"It is also under consideration by the African Union and the Security Council, but it is brought to the Assembly we will debate it."
Nearly 100 heads of state and government are expected to address the two-week long debate along with many foreign ministers.
For many from the developing world, poverty will be a major issue.
"Many countries will want to see action on the development goals put forward at the UN," said David Malone, President of the International Peace Academy in New York.
"Our concerns in the West are not necessarily priority concerns all over the world."
The reform of the Security Council is a key issue for some members who do not have one of the five permanent seats.
A meeting is planned between Japan, India, Brazil and Germany, who all want permanent seats.
However a high level panel which is discussing UN reform will only make its recommendations in December.
The resolutions passed by the General Assembly do not have any legal force, but they do have moral authority.
Analysts say it is more a forum for debate than a forum for action.