Inaugurated in 1920, the League of Nations was established by the peace treaties that ended World War I. Like its successor, the United Nations, its purpose was the promotion of international peace and security. The league was the first major international organization to attempt to tie individual nation-state security to international security. Envisioned as a collective security—rather than a collective defense—organization, the League of Nations attempted to replace individual nation-state self-interest with a shared vision of international justice and cooperation. In a first for international law, the Covenant of the League committed every signatory state to settle disputes through arbitration before going to war. The centerpiece of the League Covenant was Article 10, which bound the League to collectively preserve the territorial integrity and existing political independence of all members.

Despite its all-encompassing mandate to preserve peace and security of the international community though collective action, the League was not the first international organization to espouse such goals. The Concert of Europe (1815–1900) and the two Hague Conferences (1899 and 1907) had each in their way advanced the issue of international cooperation on the world stage, but both had significant limitations. The Concert of Europe was limited by the fact that only the Great Powers of Europe took part, and its means of conflict resolution only worked as long as each power agreed to submit. For their part, the two Hague Conferences were more inclusive (twenty-six and forty-four states, respectively) and accomplished more with respect to codifying into international law the peaceful settlement of disputes. The three Hague Conventions that arose from these conferences, along with the Permanent Count of Arbitration, presaged the Covenant of the League of Nations.

The League of Nations was a product of World War I. Many politicians believed the war had occurred in large part due to secretive diplomacy and shifting alliances between the Great Powers. For Woodrow Wilson, president of the United States from 1912 to 1920, the League of Nations represented a capstone of a morally based foreign policy; one designed to replace the dangerous balance of power politics with a more transparent and cooperative system between sovereign, democratic states governed by the principle of national self-determination. Wilson articulated his vision in his famous Fourteen Points speech presented before the U.S. Congress on January 8, 1918: "A general association of nations must be formed under specific covenants for the purpose of affording mutual guarantees of political independence and territorial integrity to great and small states alike."

As an integral part of the Treaty of Versailles, negotiated in early 1919 and enacted on January 10, 1920, the League began its operations in Geneva, Switzerland, with Sir Eric Drummond its first secretary general. Organizationally, the League consisted of a council, an assembly, and a secretariat—a structure that would subsequently serve as the model for the United Nations. The Council was originally designed to have nine members: the five great powers (Britain, France, Italy, Japan, and the United States with permanent seats, as well as four temporary rotating members (the first four were Belgium, Brazil, Greece, and Spain). However, despite an ardent cross-country campaign by Wilson, which eventually contributed to his debilitating stroke, the isolationist Senate failed to ratify the treaty and the United States never officially joined the League. Thus, the Council consisted of eight members until 1922, when two additional small states were added. In 1926 the Council was further increased to fifteen members, including Germany. The foremost goal of the League was the prevention of another world war, and the League Covenant included calls for disarmament and dispute resolution through arbitration in the International Court or inquiry before the League Council. The central focus of the Covenant was the set of articles outlining the principles and responsibilities of collective security. Article 16 declared that any state that went to war without first vetting disputes through the League processes would be deemed to have committed an act of war against all other member states of the League. Building upon the idea of deterrence through the threat of "all against one," the collective members were then expected to sever all trade and financial relations with the aggressor state. Notably member states were not legally obliged to apply military sanctions, although Article 16 claims that military sanction may be "a political and moral duty incumbent to states." The League's architects, including Wilson, believed that this collective security system would ultimately preempt the precarious alliance behavior and arms races that had caused war repeatedly in the past.

The League did enjoy many successes, particularly in settling territorial disputes such as those between Albania and Yugoslavia (1921), Germany and Poland over Upper Silesia (1922), and Greece and Bulgaria (1925). Yet the refusal of the U.S. Senate to ratify the Treaty of Versailles crippled the League from the outset. The League's structural and operational failures reflect its origins as an international organization with members of varying power and interests, the League Covenant contained structural compromises needed to ensure member state ratification and participation. The biggest of these concessions was the discrepancy between Articles 5 and 10. Article 10 promised collective preservation of territorial integrity and political independence of all member states, whereas Article 5 required all decisions taken by the League Council be made on the basis of unanimity of the members in attendance. Article 5 thus ensured a veto for any member of the Council who undertook aggressive action against another member.

Operationally the League was hampered throughout its existence by the reluctance of its member states to intervene in international disputes and apply collective security mechanisms. One of the most egregious cases was the League's weak response to Japan's invasion of Manchuria in 1931. Instead of sanctioning Japan, the League failed to take decisive action for more than seven months—partly due to the structural flaw of the League Council, which enabled Japan (as a permanent member) to delay League action. The League finally sent observers, but only after Japan formally withdrew from the League. By 1934 the League's lack of teeth would become readily evident again in the case of Italy's invasion of Ethiopia (which solicited only a weak set of economic sanctions on the part of the League) and Adolf Hitler's obvious rearming of Germany. Ultimately, the declaration of World War II—which the League was designed to prevent—spelled the demise of the institution. Although the League lingered on through the war, it finally faded into irrelevance and it functions were formally turned over to the newly created United Nations in 1945.