Isolationists in the senate opposed the treaty of versailles because

... because they opposed the United States becoming a member of the League of Nations.

The League of Nations was the signature idea of President Woodrow Wilson, point #14 of his 14 Points, an international peacekeeping association which he recommended for post-war settlements.  The Treaty of Versailles adopted that idea, 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.

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