In discussions of US foreign policy appearing in the corporate media, or among politicians or corporate media pundits, international law rarely figures. Yet (public) international law is binding on all nation states. And the international law of the use of force in international relations is not difficult to understand.
How international law works
Public international law is a system of rules, rights and obligations which applies between nations. International law generally comes from one of two sources. First, it may come from the customary practice of states: if states follow a practice, and regard it as law, then it may acquire the force of a rule of binding international law. Second, it may come from explicit treaties entered into by nation states. Most of the international law relevant to our discussion arises from both customary international law and treaty obligations.
Public international law is adjudicated by international courts, although it can also be relevant to domestic laws applied by domestic courts of any particular country. The main international court is the International Court of Justice, a UN body.
International law is a system of law: it creates binding obligations. However, there is not always a mechanism to enforce international law, since there is no "international police force". Nevertheless, international law is so essential in a globalized world that much of international trade, commerce, and interaction depends on it. Often breaches of international law are dealt with by the UN, acting through its member states, the General Assembly and the Security Council.
The General Prohibition on the Use of Force in international relations
In general, the use of force by one nation state against another is illegitimate and illegal under international law. The general prohibition on war dates back at least to the Kellogg-Briand Pact of 1928, which declared that
The High Contracting Parties solemly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another. (Art. 1)
The High Contracting Parties agree that the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means.
The general prohibition appeared in similar form in the UN Charter -- and hence, is binding upon every member state of the United Nations. Article 2 of the UN Charter declares:
3. All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered. 4. All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state...
Moreover, the UN Charter expressly obligates nation states to employ peaceful means to resolve disputes, rather than aggressive or military means. Article 33 states:
1. The parties to any dispute, the continuance of which is likely to endanger the maintenance of international peace and security, shall, first of all, seek a solution by negotiation, enquiry, mediation, conciliation, arbitration, judicial settlement, resort to regional agencies or arrangements, or other peaceful means of their own choice.
Legal uses of force
However, the general prohibition is not a complete and blanket prohibition. Force may used legally in international relations. But nation states must comply with international law, and more specifically, the UN Charter.
By the UN Security Council
The general idea is that all uses of force must be approved by a multilateral, international body -- the UN Security Council.
The provisions of Chapters VI and VII of the Charter specify how the Security Council's powers may be exercised. It is empowered to deal with any "threat to the peace, breach of the peace, or act of aggression" (art. 39), and may apply sanctions up to and including the use of "armed force... to give effect to its decisions" (art. 41)
There is one situation in which states are permitted to use force unilaterally: in self-defense.
However, states are only entitled to self-defense as an interim measure. Article 51 of the UN Charter states that:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.
Self-defense does not extend to "pre-emptive attack", or preventive war, or on the basis that a nation has weapons of mass destruction and is prepared to use them.
Controversial possible exception: humanitarian intervention
It is argued by some scholars and jurists that there is an emerging norm of customary international law permitting a nation to intervene with force in another nation to prevent a humanitarian disaster. However, this is controversial.
This potential exception might allow a state to intervene to prevent a genocide, or ethnic cleansing, for instance. The argument for humanitarian intervention under international law does not extend to broader possible reasons for intervention, such as an undemocratic government, or possession of weapons of mass destruction.
One of the arguments against the existence of a norm of humanitarian intervention is that it can be used as an excuse for any invasion, once a "humanitarian" pretext has been fabricated.
The Iraq war
Since the United States and its allies did not obtain UN Security Council authorization for the invasion of Iraq under Chapter VII of the UN Charter, the war is manifestly illegal.
Iraq did not attack the United States -- and had no intention of attacking the United States. Thus, there is no exception for self-defense.
Rather, the unprovoked attack of one nation upon another is an aggressive war, waged in flagrant violation of international law.