Complex sentence: The Committee for the Abolition of the Slave Trade, which established the Society for the Abolition of the Slave Trade in Britain in 1787, campaigned for an end to the Transatlantic slave trade from Western Africa to the New World, which Britain dominated by then.
Split into simpler sentences: The Committee for the Abolition of the Slave Trade, which established the "Society for the Abolition of the Slave Trade" in Britain in 1787. The committee campaigned for an end to the Transatlantic slave trade from Western Africa to the New World, which Britain dominated by then.