George Monbiot had an interesting piece in the Guardian a couple of days ago, in which he looked at a few cases, from the UK, Ireland and Germany, where the legality of the Iraq war, or of certain aspects thereof, came under consideration in domestic courts. (Thanks to Opinio Juris for flagging this, where Kevin John Heller has an interesting and illuminating discussion of the issues raised in the piece). In the two UK cases, involving criminal damage of military equipment to be used in the war, the jury failed to reach a verdict. In Ireland and Germany, the results were even more dramatic: in the former, protesters who had taken axes and hammers to a US Navy plane were held by a jury to have acted lawfully, while in Germany, a court threw out a charge of insubordination against an officer who had refused to carry out an order he felt would implicate him in the war in Iraq, on the basis that the war was indeed illegal.
The material background of the cases involved can be found in the House of Lords judgment in R v. Jones (March 2006). This was the case in which it was decided that the defendants could not rely on the fact that state agression is a crime under international law in constructing their defences before the UK courts. This, of course, deals with the issue of whether or not customary international law forms part of the common law of England and Wales, which was raised, for example, by Lord Millett in his separate judgment in the Pinochet case. In Jones, I think, we have a fairly definitive statement of the current UK law on this position: that although customary international law may be viewed as having been assimilated into domestic law even in the absence of specific legislative action, this process cannot be viewed as happening either necessarily or automatically (and, interestingly, the court here explicitly distances itself from Lord Millett's view, expressed in Pinochet, that international torture became a crime under domestic UK law as soon as it became a part of customary international law - see para. 19).
Indeed, the Court's judgment seems to make it very unlikely that any non-legislated incorporation of new customary international law into English domestic law will be recognised ever again, in terms of criminal law at the very least. The relevant passage from Lord Bingham's judgment reads as follows (para. 28):
The lack of any statutory incorporation is not, however, a neutral factor, for two main reasons. The first is that there now exists no power in the courts to create new criminal offences... While old common law offences survive until abolished or superseded by statute, new ones are not created. Statute is now the sole source of new criminal offences. The second reason is that when it is sought to give domestic effect to crimes established in customary international law, the practice is to legislate. Examples may be found in the Geneva Conventions Act 1957 and the Geneva Conventions (Amendment) Act 1995, dealing with breaches of the Geneva Conventions of 1949 and the Additional Protocols of 1977; the Genocide Act 1969, giving effect to the Genocide Convention of 1948; the Criminal Justice Act 1988, s 134, giving effect to the Torture Convention of 1984; the War Crimes Act 1991, giving jurisdiction to try war crimes committed abroad by foreign nationals; the Merchant Shipping and Maritime Security Act 1997, s 26, giving effect to provisions of the United Nations Convention on the Law of the Sea 1982 relating to piracy; and sections 51 and 52 of the International Criminal Court Act 2001, giving effect to the Rome Statute by providing for the trial here of persons accused of genocide, crimes against humanity and war crimes, but not, significantly, the crime of aggression.
Lord Hoffmann concurred, noting simply "the democratic principle that it is nowadays for Parliament and for Parliament alone to decide whether conduct not previously regarded as criminal should be made an offence" (para. 60). He goes even further, in a interesting passage of dicta on "the limits of self-help" (paras. 70-88), holding that allowing citizens to use force against military installations on the basis purely of their own views - however honestly held - of the legality of state action would turn the Statute of Treason into a dead letter; instead, legal and democratic means must be used to force a change in government policy. Hence, he concludes that the defendants should be convicted "even if aggression had been a crime in domestic law" (para. 87). It seems, however, that not enough members of the jury in the most recent case were in agreement...