In 1839, the British occupied the port of Aden and established it as a colony in September of that year. They also set up a zone of loose alliances (known as protectorates) around Aden to act as a protective buffer. North Yemen became independent of the Ottoman Empire in 1918 and became a republic in 1962. In 1967, the British withdrew and gave back Aden to Yemen due to extreme pressure of battles with the North and Egyptian allies. After the British withdrawal, this area became known as South Yemen. The two countries were formally united as the Republic of Yemen on May 22, 1990.