One could argue that this statement is true. In the United States, the international slave trade was outlawed in 1808. Along with this, the 13th amendment made slavery illegal as of 1865.
France got rid of slavery right before the 19th century began, as it became illegal after 1794. Slavery also became illegal in Great Britain during the 19th century. Slavery in Britain was outlawed as of 1833. These three examples show the decline of slavery in the 19th century.
