The law is the easy part.
Absent agreement between them, the law between states can be found in Conventions and, where even they are absent, reasons of justice. Since 1945 however we have had the United Nations Charter, that was created by agreement. In areas it covers, it provides us with the positive law, replacing what was there before. The central point of the positive law is that it gives us guidance independent of reasons of justice. It closes off appeals to them. If it didn’t, the positive law would have little point.
Article 2(4) of the UN Charter has a prohibition on the use of force. This allows of two exceptions. The first is in Chapter VII, where the Security Council has been given power in order to fulfill its responsibility for the maintenance of international peace and security. The second is the right to self defence in article 51. (I argued back in 2015 that the drone strikes against ISIS targets were justified on the basis of the defence of the state of Iraq.)
Take an extreme example. Say a government of a nation state led by a brutal dictator was carrying out a campaign of genocide against a minority group. Let it be assumed that it can be proven beyond peradventure that dropping a bomb on an airfield, hurting nobody, could prevent this. One permanent member of the security council opposes this action. Can other countries, individually or collectively, drop the bomb saving millions of lives?
As a matter of the positive law, the answer is clearly no. There is no ‘humanitarian intervention’ exception in the text of the Charter. Maybe such a rule could have been justified absent the Charter, but it now covers the field. There is no way of interpreting the words as meaning anything other than what they say.
That the Charter allows for no such exception is apparently astonishing given the context of its drafting. However, in 1945 it was envisaged that such action would be taken collectively. There is no get out where one permanent member of the security council opposes intervention.
It follows from the two propositions that international law on the use of force is found in the Charter, and that the Charter by its words only permits the use of force in the two situations described, that I consider that the bombing in 1999 by Nato forces in order to (successfully) prevent the ethnic cleansing of thousands of Albanians as contrary to the positive law.
That does not, however, answer the moral question of whether it was wrong.
Governments have good reasons for complying with international law.
First, that is what they have agreed to do. Pacta sunt sevanda: agreements are to be kept. This is principle of justice that applies to states as much as to people. States agreed to the UN Charter, and if they do not like certain features of it then the correct course is to vary it through further agreement.
Second, breaking international law undermines respect for it. Why should other states comply with international law if we do not? Unlike domestic law, international law is (largely) free of any sanctions for breach.Without an enforcement mechanism, the only way the United Kingdom has of encouraging others to comply with their treaty obligations is for us to comply with ours. The absence of a strong regime of sanctions for breach means that the United Kingdom has a stronger, not weaker, moral reason for compliance.
Third the moral question of whether to intervene in cases such as Syria is often extremely fraught and difficult. Dropping bombs may deter the illegal use of chemical weapons, but such illegality by the Syrian state does not alone provide other states a legal privilege to drop bombs upon it. On the other side, dropping bombs kills people, many innocent. There is a danger of escalation. It would be better if the question of when action is permitted and required were settled by rules drawn up dispassionately in advance, and that is what the UN Charter tries to do.
But although the positive law has great weight in our moral reasoning, it does not have infinite weight. Sometimes breaking the law is the right thing (morally) to do. Two facts lessen the weight international law has.
First, it is obviously deficient, as my extreme example of the single bomb to prevent genocide illustrates. The bar to collective action is set too high. One permanent member of the UN Security Council having power to block any collective action entails in practice and not just in theory that no legal force will be used in some cases to prevent even the most grotesque crimes committed within nation states.
The explanation, but not justification, for the high bar is realpolitik, not justice, Members of the security council wanted, and want, the power to block the lawful exercise of force against themselves or allied countries. There is no practical prospect of this being changed. In this respect, international law is in a worse state than it was before 1945 as a matter of justice.
Second, the question of whether to comply or not with international law is an example of the prisoners’ dilemma, All states will be better off if there is universal compliance, but if some states follow the rules, whilst others do not (by for example annexing part of a neighbouring state) that may place those who comply with the rules in a worse position than they otherwise would be.
UK Government’s Legal Position
The UK government sets out its legal position as follows
The legal basis for the use of force is humanitarian intervention, which requires three conditions to be met:
(i) there is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief;
(ii) it must be objectively clear that there is no practicable alternative to the use of force if lives are to be saved; and
(iii) the proposed use of force must be necessary and proportionate to the aim of relief of humanitarian suffering and must be strictly limited in time and in scope to this aim (i.e. the minimum necessary to achieve that end and for no other purpose).
There is no textual support for this within the UN Charter at all. It is implausible as a statement of the positive law.
It is however a good statement of what the law ought, as a matter of justice, to be. There is a danger of judging these things in hindsight. The bombing of the former Yugoslavia “worked” (whereas other interventions have not) but my judgement, for what that is worth, is that it was right but unlawful, regardless of the outcome.
The UK government has an interest in respect for the rule of law, both nationally and internationally. It cannot say “this action is right, albeit unlawful.” So, it portrays what is in fact an argument of justice as one of legality.
Whether the decision to bomb is, morally, the correct one cannot be definitively answered for us by the positive law, Each of us, in this regard, is on our own.