Actually that's not true
Dozens of countries, including the possessions of British, French, and Spanish empires ended slavery peacefully during the late eighteenth and nineteenth centuries. Only in the United States was warfare associated with emancipation. There was violence in some other countries during the abolition of slavery, but as Fogel and Engerman point out,"In countries such as Colombia and Venezuela the emancipation of slaves became an instrument of the revolutionaries who sought state power"; it was not motivated by a desire for emancipation per se.--The Real Lincoln, DiLorenzoColumbia and Venezuela went through the same thing, however in their cases, it was seen as an attempt by revolutionaries to gain power and change the direction of the nation.