The ideas behind the League represented a fundamental shift in thought from the preceding hundred years; rather than each nation pursuing its national interests at the expense of others or maintaining through Realpolitik and a balance of powers, the League brought in the concept of international co-operation. The League was structured under three main constitutional organs: the Assembly (all members of the League); the Council (the executive body of the League), and the Permanent Secretariat. These bodies would later be carried on to the United Nations.
To its credit, the League successfully mediated several disputes, especially surrounding borders, the opium trade, and slavery. For example, Sweden and Finland had a long-running argument over the Aland Islands that brought the two nations to the brink of war by 1920. Britain asked the League to intervene, and the agreement forged by the League in that dispute (that the islands would remain under Finnish control but demilitarized) remains in force to this day. It also successfully halted the use of slave labor on Liberia rubber plantations owned by Firestone.
However, the League ultimately had several structural weaknesses that handicapped its ability to enforce peace.
Even though the League was the brainchild of Woodrow Wilson, the U.S. never joined, much to Wilson's embarrassment. The main sticking point was Article X, which established collective security. Opponents, led by Senator Henry Cabot Lodge, believed this would undermine U.S. sovereignty, namely Congress's constitutional right to declare war, and went so far as to claim the League was precisely the type of "entangling alliances" that George Washington, Thomas Jefferson, and James Monroe had wanted the U.S. to avoid. Lodge had annoyed at how he and his fellow Republicans had been excluded from the Paris Peace Conference and the drafting of the League, and would be more open to the idea if Wilson had been more flexible.
However, Wilson refused to compromise on Article X, which he saw as essential to guarantee world peace. This left Wilson and Congress at an impasse, resulting in the League going nowhere. Wilson's successor, Warren Harding, was opposed to the League, so any hope of U.S. membership died with his election. Naturally, the noninvolvement of the U.S. would lead to future problems for the League.
The League lacked an armed force and depended on the victorious Great Powers of the Great War-Great Britain, France, Italy, and Japan-to enforce its resolutions via military or economic pressure. The concept of collective security and disarmament, central to the League's goals of peace, failed as the member nations were unwilling to disarm their militaries for varying reasons. Furthermore, as Those Wacky Nazis re-militarised their army and threatened peace in Europe, the surrounding countries opted to follow suit.
Finally, the invasion of Ethiopia by Fascist Italy showed just how impotent the League was since none of its members sought to lift a finger to stop the invasion despite each member being obligated to deter Italy's aggression under the principles of collective security. The inaction of the League emboldened Italy and Nazi Germany, and a betrayed Emperor, Haile Selassie, would go before the League and proclaim that the failure of collective security would spell doom for the world.
After World War II broke out, the League was left dormant for several years and officially dissolved by its assembly in 1946. Most of its assets and documents were given to the fledgling United Nations. While the League was a failure, it was considered by some to be an excellent first try in building international institutions around cooperation rather than conquest.
References in fiction:
- The penultimate episodes of Star Trek: Enterprise introduced the Coalition of Planets, a much looser alliance that served as the precursor to the United Federation of Planets prior to the Romulan War. The writers clearly used the League as an inspiration.
- At one point, WWE featured a heel stable titled the League of Nations comprised of Sheamus, Rusev, Alberto Del Rio, and Wade Barrett. The faction eventually broke apart in its final weeks by kicking Barrett out and then leaving the ring one by one in their final match, resulting in Sheamus shouting, "The League is finished!"
- Wilson, a 1944 Biopic about Woodrow Wilson, naturally climaxes with Wilson's failed campaign to get the U.S. to join the League of Nations. This being a wartime film, it's very much a plea for the U.S. not to screw it up again come the end of World War II.