The Senate did not approve the Treaty of Versailles, because on approval, the US would be part of the League of Nations, which would put the US in events that they did not want to take part in.
Remember that back in the day, the US was inside the Isolationist Age, in which the US only backed such that had their own interest, and tried not to take part in world affairs. This ended at the end of WWII, in which the US became a global power, and saw that they must take leadership in the world.
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