Question

The fear that the United States would be drawn into costly foreign wars was the main reason that the US Senate rejected:

Answer 1

Answer:  The Treaty of Versailles -- which included provision for the League of Nations.

Explanation:

The main reason that the US Senate refused to ratify the Treaty of Versailles was it would mean the US would enter into the League of Nations.  Senators believed  that doing so meant giving up some of the United States' own sovereignty and could commit the US to defend other nations' security rather than its own.  The Senators feared the US would be drawn into costly foreign wars if they committed to the terms of the Treaty of Versailles.   Thus, the United States never joined the League of Nations, in spite of the fact that an organization such as the League of Nations was the signature idea of US President Woodrow Wilson.  He had laid out 14 Points for establishing and maintaining world peace following the Great War (World War I).  Point #14 was the establishment of an international peacekeeping association.

The Treaty of Versailles adopted Wilson's idea and called for the creation of the League of Nations.  But back home in the United States, there was not support for involving America in any association that could diminish US sovereignty over its own affairs or involve the US again in wars beyond those pertinent to the United States' own national security.   Because of its objections to membership in the League of Nations, the United States Senate refused to ratify the Treaty of Versailles.