There is broad agreement that America has an inherent right in customary international law to defend itself against the sort of attacks it sustained on September 11th. Article 51 of the UN Charter recognises this right of self-defence, as did the International Court of Justice in 1986 when the US placed mines in Nicaragua's ports.
This right of self-defence is a strictly defined exception to the rule on which international order is founded: the non-use of force. It is this rule which protects the existence of small states such as Ireland from the militarily powerful.
Three elements must be respected if the use of force, in exercise of the right to self-defence, is to be exercised lawfully in the current context, bearing in mind the novelty of asserting the right where the attack is by terrorists, rather than by another state.
The response must be both necessary and proportionate to the attack (the use of force against another state for revenge, to punish or to reassure public opinion is unlawful) and, third, it is for the international community as a whole, particularly the Security Council to assess whether these criteria of necessity and proportionality are met.
This is at the heart of Ireland's crucial role in this crisis.
In the past, the test of necessity has been met when there has been insufficient time for deliberation because there is an on-going armed attack. Here, the nature of the attack was different from the classical inter-state situation. Any threat of further attacks in the US from "sleeper" terrorists is most directly met by increased internal security and intelligence.
The question therefore is whether an attack on Afghanistan is necessary for the self-defence of the US. If the party responsible for the attack on the World Trade Centre is not the government of a state from which the terrorists operate, the question arises as to whether the use of force against that state is lawful?
Issues include the question of the evidence establishing legal responsibility for this attacks. The USA is refusing to provide evidence to Afghanistan, or to the Security Council, regarding Osama bin Laden's alleged involvement in the September 11th attacks.
An argument can be made that the right of self-defence applies if the government of Afghanistan is knowingly harbouring those accused of an international crime against humanity. However, can Afghanistan be knowingly harbouring this alleged criminal if the case he has to answer has not been produced?
If it can, such logic might justify the bombing of suspected arms locations in Ireland by the UK if it felt Ireland were not doing enough against terrorist attacks in Northern Ireland. The issue of evidence is central to the accountability of states. They must justify any use of force (including in self-defence) to the rest of the international community.
The attacks on New York and Washington - crimes against humanity- demonstrate the rationale for the future International Criminal Court. . Since 1998, 139 states have signed the treaty wich would establish the Court. Pakistan and Afghanistan are not among them.
Convincing them and others to become parties should be an urgent priority of Ireland and the Security Council. Thirty-seven of the 60 ratifications needed to create the Court have been lodged - neither the US nor Ireland is among them. The US has demanded that Afghanistan hand over Osama bin Laden while also speaking of hunting him down dead or alive. If the permanent International Criminal Court had already been operating, it might conceivably have made this a more palatable option for Afghanistan by providing the prospect of a fair trial - difficult to achieve in the US in the current climate.
In the past, the Security Council has worked to facilitate the surrender of suspected terrorists for prosecution. Sanctions were imposed against Libya to induce the surrender of suspects in the Lockerbie aircraft bombing.
Sanctions were similarly imposed on Afghanistan to induce the handing over of bin Laden for the East African Embassy bombings. No effective coalition was formed for the peaceful handing over of the suspect for trial on those charges. Would such a coalition have been successful if evidence of his responsibility for the September 11th attack were adduced, and a credible venue found for a fair trial as in the Lockerbie case?
In this light, can it be said that all other means short of force been exhausted?
If it is shown in answer to questions such as those above, that an attack on Afghanistan is necessary for the self-defence of the US, the question of proportionality arises.
There is much speculation as to the nature of the force which the US is planning. Proportionality requires that the aims be strictly confined to removing the actual threat to the US by bringing to justice those alleged to be legally responsible. The Security Council has stated that bringing the perpetrator's to justice is the appropriate response. Intervention in Afghanistan's civil war to replace the government would in principle be unlawful interference in the internal affairs of another state, and, if it is not justified in self-defence some would argue that the attack would constitute a war of aggression, an international crime (whose definition remains controversial).
In addition to defining the aim, proportionality requires that once force is used, the laws of war apply to protect non-combatants. For example, the International Court of Justice has made it clear that states do not have unlimited freedom of choice in the weapons they use.
In a context where humanitarian organisations have warned that one foreseeable consequence of a US attack on Afghanistan is that several million people will become refugees, it is for the US to bring evidence as to how is this necessary to the defence of the US? How is it proportionate to achieving its self-defence?
Of central importance is the process by which any state using force against another accounts for and justifies its actions in light of the legal standards that apply. In this regard the Security Council has a leading role.
In the last 20 years, the US has proposed, controversially, that its inherent right of self-defence can be triggered if a US national is attacked abroad. Even if an attack on a national abroad could lead to a lawful exercise of a state's right of self-defence, the US went a step further in 1993 when it attacked Iraq and argued that it could attack another state to protect its nationals from a potential future attack. The principle of the non-use of force in international relations is gravely undermined by such an expansive precedent.
This is also true if the US refuses to be accountable to the Security Council for its use of force. When the US bombed both Afghanistan and Sudan, the evidence which it said it had at the time regarding those state's complicity in the American Embassy bombings in East Africa, was never produced.
Ireland, among others, failed to insist on evidence being made available in advance of the bombings, or even within a certain time limit after they had been carried out. The bombings are now widely seen to have been neither clearly justified nor effective.
The US will be the first to gauge what is "necessary" and "proportionate" by way of a response to the attack it has suffered. Clearly, the rule of international law requires that no one should be judge in their own cause otherwise it is the law of the powerful which prevails. What is "necessary" or "proportionate" is for the international community as a whole, and specifically for the Security Council to assess.
Article 51 of the UN Charter requires that measures taken by the US in self-defence "shall be immediately reported to the Security Council" and shall not diminish the responsibility of the Council to take any action it deems necessary for international peace and security.
The US has not shown the intention of respecting the role of the Security Council in assessing what is necessary and what is proportionate - that is, what is lawful. How will Ireland ensure that it is not assisting in either unnecessary or disproportionate actions?
Sidelined from decision-making which the rule of law requires, the only role which the UN is automatically expected to fulfil is to mop up the humanitarian disaster which has already started.
This, as well as unpredictable destabilisation of Afghanistan's neighbours and the Middle East is a threat to international peace and security. Under the UN Charter, the maintenance of international peace and security is the "primary responsibility" of the Security Council - and so Ireland now bears an enormous responsibility to ensure the role is fulfilled. Failure to hold each state to the rule of international law undermines the security of all states.
Ireland has learned painfully that the root causes of terrorism lie in human rights violations. It is only when these violations are addressed that a long term solution to terrorism can be built. The repressive regimes of the Arab world encourage their anti-American and anti-Israeli press, stoking the fires of extremism to divert attention from their internal repression.
The US's record of support for Israel's continuing security "non-solution" to its security crisis - its attacks on civilians, impunity for crimes such as the massacres at the Sabra and Chatila refugee camps in Lebanon - all directly fuel the Islamic extremism which leads to terrorism, undermining the security of the US, and us all.
As the UN Secretary-General put it in 1998: "Today's human rights violations are the causes of tomorrow's conflicts."
Ireland should play a considered role in combating terrorism. It is in a position to speak with authority. Ireland should first make clear that its support of US actions is conditional on compliance with international law, and make clear what it understands that to mean. One element is that Ireland, and the Security Council should have sufficient information throughout to comply with their responsibilities.
Ireland should ratify urgently the Statute of the International Criminal Court and seek the ratification of it by others. It should advocate that the US pays the rest of its UN membership dues - one of the most under-funded functions of the UN is its work to help address human rights.
Ireland now has an historic opportunity to help ensure that the international law which states have themselves created to govern their conduct should be upheld.
We have a track record to live up to: from the late Frank Aiken as Minister for External Affairs and the diplomacy of Conor Cruise O'Brien. As Ireland's Ambassador to the UN, Richard Ryan, has recently put it: "Ireland, like most members, attaches huge importance to the authority of the UN as their recourse for problems and crises. At the heart of the UN is the moral, legal and political authority of the Security Council."
The perception of Ireland around the world would be affected if it does not hold all states accountable to law, and the hope of many that Ireland would play a constructive, principled role on the world stage, undermined.
For the next month, the manner in which Ireland plays its role as President of the Security Council is a matter on which not only that heritage, but innocent lives and the interests of small states depends.
Karen Kenny is an expert member of the Department of Foreign Affairs-NGO Committee on Human Rights and was human rights specialist with UN human rights operations in Rwanda, El Salvador, Bosnia and Herzegovina. She is co-founder of the International Human Rights Trust.