Complex sentence: Portugal however was one of the largest slave trading nations was Britains ally, so in 1810 under diplomatic pressure, a convention was signed allowing British ships to police Portuguese shipping meaning it could only trade in slaves from its own African possessions.
Split into simpler sentences: Portugal however was one of the largest slave trading nations and Britain's ally. So in 1810 under diplomatic pressure, a convention was signed allowing British ships to police Portuguese shipping meaning it could only trade in slaves from its own African possessions.