The United States did not join the League of Nations mainly because the U.S. Senate refused to ratify the Treaty of Versailles, which included the Covenant of the League. The core issues were concerns about sovereignty, entangling alliances, and American constitutional checks on war powers. Key points

• Sovereignty and entanglement: Critics argued that joining the League would cede congressional power over declaring war to an international body and could drag the U.S. into conflicts without explicit congressional approval. This worried many lawmakers who favored retaining decisive control over U.S. military commitments.

• Reservations and opposition in the Senate: A faction known as the Irreconcilables opposed the League on principle, while others favored reservations to protect U.S. sovereignty and constitutional authority. Despite Wilson’s leadership and support for the Covenant, the Senate, led by Henry Cabot Lodge, blocked ratification and any version lacking sufficient protections.

• Wilson-Lodge conflict: Personal and political battles between President Woodrow Wilson and Senator Lodge hindered compromise. Wilson resisted the proposed reservations, arguing for ratification as negotiated, while Lodge and others pressed for amendments to safeguard U.S. interests. The deadlock ultimately prevented U.S. accession.

• Historical context of American sentiment: The United States had a long-standing preference for avoiding entangling alliances and excessive involvement in European affairs, a view rooted in earlier Republican and at times isolationist traditions. This broader mood amplified opposition to the League.

Additional nuances

• Wilson’s Fourteen Points and moral leadership: Wilson framed a vision of a just international order, with the League as a central instrument, yet domestic political realities prevented adoption of the terms he championed. The gap between idealism and congressional approval was a decisive factor.

• Aftermath and effects: The U.S. never became a formal member, but American diplomacy and individuals interacted with the League in various ways. The absence of the United States altered the League’s dynamics and contributed to its eventual vulnerabilities in the 1930s.

Direct answer
The U.S. did not join the League of Nations because the Senate rejected ratification of the Treaty of Versailles, largely due to concerns over sovereignty and being bound by collective security, opposition from isolationist and reservations-minded lawmakers (notably Senator Henry Cabot Lodge), and a broader American reluctance to commit to entangling alliances. The resulting failure to secure a two-thirds Senate majority left the United States outside the League for its entire existence.