Philippe Sands: So this is the real reason the generals are up in arms

In part it seems that it is the use of the words "war crime", apparently a first in the prosecution of British soldiers. Yet the concept of war crimes is not new in English law. The 1949 Geneva Conventions codified the laws of war, including war crimes relating to the mistreatment of detainees. The 1949 Conventions were largely implemented into English law by the Geneva Conventions Act of 1957, and have long governed Britain's military activities.

More recently the Statute of the International Criminal Court (ICC), in force since 2002, has listed the four categories of international crimes: war crimes, crimes against humanity, genocide and the crime of aggression (waging illegal war).

The ICC was created to end impunity, given broad recognition that the enforcement of many of the pre-existing rules was grossly inadequate. So the principle was put in place that prosecutions for war crimes and the other international crimes should occur at the national level. The ICC would exercise jurisdiction only if there were no good reasons to explain a failure of a national prosecution. This is known as the principle of "complementarity".

This is the background against which last week's announcement took place. Some of the prosecutions are based on the International Criminal Court Act of 2001, the first time that Act has been used in this way. It is not clear whether the prosecutions were motivated by any fear that the ICC could step in: in his full legal advice of 7 March 2003 the Attorney General advised the Prime Minister that "it is likely that the [ICC] will scrutinise any allegation of war crimes by UK forces very closely".

It is right that the laws of war should be enforced against those who are alleged to have violated them in this country and abroad. By enforcing its laws the United Kingdom makes it more likely that the rules will be respected and that they will be applied elsewhere.

There can be no criticism of any decision to prosecute a British soldier for war crimes if the facts indeed justify a prosecution. What is crucial, however, is that the international criminal rules should be applied to all, irrespective of status or seniority. For many it was ironic that this week's announcement was made by the Attorney General, whose consent was required for prosecution under the 2001 Act. It was the Attorney General, of course, who signed off on the legality of the war, despite the fact that it was considered to be illegal by most governments and commentators, as well as the Foreign Office legal adviser and the UN Secretary General.

In the scale of illegalities, those alleged to have been occasioned by the members of the Queen's Lancashire Regiment pale in comparison with that unleashed by the Prime Minister and his Attorney.

An illegal war can constitute the crime of aggression. Happily for the Attorney and the Prime Minister the ICC does not yet have jurisdiction over the crime of aggression. Nor has that crime been incorporated into English law by the 2001 Act.

But, as the Attorney correctly pointed out in a little noticed passage in his 7 March 2003 advice, "aggression is a crime under customary international law which automatically forms part of domestic law". On the Attorney's own view those most closely associated with the "crime of aggression" could themselves also be subject to investigation or prosecution.

Yet, as Balzac put it: "Laws, like the spider's web, catch the small flies and let the larger ones to go free."

Philippe Sands QC is Professor of Law at University College London and a barrister at Matrix Chambers. He is the author of 'Lawless World: America and the Making and Breaking of Global Rules' (Penguin)