The united states senate refused to ratify the treaty of versailles after ww1 because many senators believes?

The main reason for the Senate's rejection of the treaty was to avoid the "foreign entanglements" of the League. However, a more sinister reason might have been that the Republican dominated Senate disliked President Woodrow Wilson (who refused to have any Republican leaders with him on the trip to Versailles) and wanted his plans for the treaty and the League to fail.

Many senators "were concerned about future United States obligations in foreign affairs."

No single ruler united the many city-states into one empire.