Why the Northern States of the USA, argued for ending slavery ?
The northern states of the USA, where the economy did not depend on plantations (and therefore on slavery), argued for ending slavery which they condemned as an inhuman practice.
In 1861-65, there was a war between the states that wanted to retain slavery and those supporting abolition. The latter won. Slavery was abolished, though it was only in the twentieth century that the African Americans were able to win the battle for civil liberties, and segregation between 'whites' and 'non-whites' in schools and public transport was ended.
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(M. Imp.)