The League of Nations had been formed in 1919, and the final version of the Covenant of the League of Nations became Part I of the Treaty of Versailles, and could only begin to function, formally and officially, after the Peace Treaty of Versailles came into effect. Thus, the League of Nations was not officially inaugurated until January, 1920.
The 32 original Members of the League of Nations were also Signatories of the Versailles Treaty. In addition, 13 other States were invited to accede to the Covenant. The League of Nations was open to all other States, providing they fulfilled certain requirements. At its greatest extent, from September 1934 to February 1935, it had 58 members.
The League was the first international organization whose principal mission was to maintain world peace. Its primary goals, as stated in its Covenant, included preventing wars through collective security and disarmament and settling international disputes through negotiation and arbitration. Other issues in this and related treaties included labor conditions, just treatment of native inhabitants, human and drug trafficking, the arms trade, global health, prisoners of war, and protection of minorities in Europe. At its greatest extent from 28 September 1934 to 23 February 1935, it had 58 members.
The League was marked by notable failures, most glaringly, in preventing the invasion of Manchuria by Japan, the annexation of Ethiopia by Italy, and the onset of World War II. The powerlessness of the League contributed to the alienation from it by the Member States.
It did have a number of successes, however, including cooperative ventures that were transferred to the United Nations.
At the 1943 Tehran Conference (a strategy meeting of Joseph Stalin, Franklin D. Roosevelt, and Winston Churchill from November 28 to December 1, 1943), the Allied powers agreed to create a new body to replace the League: the United Nations. Many League bodies, such as the International Labour Organisation, continued to function and eventually became affiliated with the UN. The designers of the structures of the United Nations intended to make it more effective than the League.
The final meeting of the League of Nations took place on April 18, 1946 in Geneva. This session concerned itself with liquidating the League: it transferred assets to the UN, returned reserve funds to the nations that had supplied them, and settled the debts of the League. Robert Cecil, a British lawyer, politician and diplomat and one of the architects of the League of Nations, said:
Let us boldly state that aggression wherever it occurs and however it may be defended, is an international crime, that it is the duty of every peace-loving state to resent it and employ whatever force is necessary to crush it, that the machinery of the Charter, no less than the machinery of the Covenant, is sufficient for this purpose if properly used, and that every well-disposed citizen of every state should be ready to undergo any sacrifice in order to maintain peace … I venture to impress upon my hearers that the great work of peace is resting not only on the narrow interests of our own nations, but even more on those great principles of right and wrong which nations, like individuals, depend.
The League is dead. Long live the United Nations.”
The Assembly passed a resolution that “With effect from the day following the close of the present session of the Assembly [i.e., April 19], the League of Nations shall cease to exist except for the sole purpose of the liquidation of its affairs as provided in the present resolution.”