The first international instrument outlawing war was the Treaty Providing for the Renunciation of War as an Instrument of General Policy (also known as the Kellogg-Briand Pact) in 1928.
[The High Contracting Parties]
Deeply sensible of their solemn duty to promote the welfare of mankind;
Persuaded that the time has, [sic] 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 [sic] 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 endeavour 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;
Have decided to conclude a Treaty and for that purpose have appointed as their respective Plenipotentiaries (…) who, having communicated to one another their full powers found in good and due form have agreed upon the following articles:
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.
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…