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 additional States were invited to accede to the Covenant. The League of Nations was open to all other States, providing they fulfilled certain requirements.
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.