Final answer:
The U.S. Senate rejected the Treaty of Versailles predominantly due to concerns over joining the League of Nations, which many senators believed would undermine U.S. sovereignty and draw the country into unwanted foreign conflicts. Other factors included the Senate's exclusion from the peace process and fears of substantive obligations under Article X of the League's covenant.
Step-by-step explanation:
The main reason the U.S. Senate rejected the Treaty of Versailles was concerns regarding the League of Nations. Senators were particularly worried that membership in the League would draw the United States into foreign disputes and thus infringe upon their constitutional power to declare war. This was a significant point of contention against the treaty, as it implied a loss of sovereignty in determining when the nation would engage in military conflicts.
Additionally, the Senate harbored dissatisfaction with the way President Woodrow Wilson handled the peace process, failing to consult key senators. Furthermore, there was a broader fear that the country would be obligated to participate in foreign conflicts, a scenario many wished to avoid. The formation of the League of Nations was seen as a potential threat to U.S. autonomy, where Article X was particularly controversial, suggesting a binding commitment to mutual defense among member nations which was unacceptable to many senators.