Complex sentence: For a brief period in the 1790s the British attempt to establish a rival foothold on an offshore island, at Bolama, but by the 19th century the Portuguese are sufficiently secure in Bissau to regard the neighbouring coastline as their own special territory.
Split into simpler sentences: For a brief period in the 1790s the British attempt to establish a rival foothold on an offshore island, at Bolama. By the 19th century the Portuguese are sufficiently secure in Bissau to regard the neighbouring coastline as their own special territory.