Complex sentence: The British had entered into a treaty with Spain eliminating the slave trade south of the equator and saw it as a matter of International Law that the United States release the Africans applying diplomatic pressure to achieve this including invoking their own treaty with the United States.
Split into simpler sentences: The British had entered into a treaty with Spain eliminating the slave trade south of the equator and saw it as a matter of International Law that the United States release the Africans. They applied diplomatic pressure to achieve this including invoking their own treaty with the United States.