Treaty for the Renunciation of War

The High Contracting Parties solemnly 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.

From the Kellogg-Briand Pact (also known as the General Treaty for the Renunciation of War, or the Pact of Paris)

The Kellogg-Briand Pact was signed by the US, UK and 13 others in 1928, and then later on by 47 further countries (including such currently relevant ones as the Kingdom of Serbia, Afghanistan, Persia and Venezuela). The treaty contained no methods of enforcement, and has for that reason been described by some as ineffective, but it actually played a central role at the Nuremberg Trials, and was commonly cited in deliberations concerning the the crime of aggression ('crimes against peace'). In its final judgement, the Tribunal argued:

The nations who signed the Pact or adhered to it unconditionally condemned recourse to war for the future as an instrument of policy, and expressly renounced it. After the signing of the Pact, any nation resorting to war as an instrument of national policy breaks the Pact. In the opinion of the Tribunal, the solemn renunciation of war as an instrument of national policy necessarily involves the proposition that such a war is illegal in international law; and that those who plan and wage such a war, with its inevitable and terrible consequences, are committing a crime in so doing. War for the solution of international controversies undertaken as an instrument of national policy certainly includes a war of aggression, and such a war is therefore outlawed by the Pact.

From The Nuremberg Judgment (emphasis added)

Incredible though it might seem, in signing this treaty in 1929 the UK thereby expressly renounced war with other contracting parties to this treaty - including with Afghanistan and Serbia. The UK solemnly agreed that the settlement of disputes with treaty signatories should never be sought except by pacific means. And just for the record - neither Serbia nor Afghanistan broke the terms of the treaty first.

As far as Iraq is concerned, having created the country, the UK was in control at the time of the treaty - so Iraq is not a co-signatory. I wonder what the legal niceties are here: no doubt Lord Goldsmith could construct an argument to the effect that Britain had not signed any treaty with its constituent or colonial parts, so it might do what it liked with them. But in any case, and from the quote above, it appears that the Tribunal took the view that not only was war between treaty co-signatories illegal, but (aggressive) war was illegal per se - and that signing the treaty was an acknowledgement of that fact.

Kellogg-Briand Pact 1928

Treaty signed at Paris, August 27, 1928; proclaimed, July 24, 1929 (and never revoked).

(Extracts only. For the whole treaty and list of signatories see Kellog-Briand Pact)

Deeply sensible of their solemn duty to promote the welfare of mankind;

Persuaded that the time has, come when a frank renunciation of war as an instrument of national policy should be made to the end that the peaceful and friendly relations now existing between their peoples may be perpetuated;

Convinced that all changes in their relations with one another should be sought only by pacific means and be the result of a peaceful and orderly process, and that any signatory Power which shall hereafter seek to promote its national interests by resort to war a should be denied the benefits furnished by this Treaty;

Hopeful that, encouraged by their example, all the other nations of the world will join in this humane endeavor and by adhering to the present Treaty as soon as it comes into force bring their peoples within the scope of its beneficent provisions, thus uniting the civilized nations of the world in a common renunciation of war as an instrument of their national policy;

...

ARTICLE I

The High Contracting Parties solemnly 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.

ARTICLE II

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...