The area that comprises Yemen today has traditionally, since the nineteenth century, been divided in two along a north-south divide. What was once North Yemen was part of the former Ottoman Empire during the nineteenth and early twentieth centuries, while what used to be South Yemen fell under British influence starting in 1839 when the British captured the port of Aden. After the collapse of the Ottoman Empire in 1918, North Yemen became an independent republic while South Yemen continued under the British until Britain withdrew in 1967. British authorities left southern Yemen in November 1967 in the wake of an intense terrorist campaign. The two countries were formally unified as the Republic of Yemen in 1990.