North Yemen became independent of the Ottoman Empire in 1918. The British, who had set up a protectorate area around the southern port of Aden in the 19th century, withdrew in 1967 from what became South Yemen. Three years later, the southern government adopted a Marxist orientation. The massive exodus of hundreds of thousands of Yemenis from the south to the north contributed to two decades of hostility between the states. The two countries were formally unified as the Republic of Yemen in 1990. A southern secessionist movement in 1994 was quickly subdued. In 2000, Saudi Arabia and Yemen agreed to a delimitation of their border. Fighting in the northwest between the government and Huthi rebels, a group seeking a return to traditional Zaydi Islam, began in 2004 and has since resulted in six rounds of fighting - the last ended in early 2010 with a ceasefire that continues to hold. The southern secessionist movement was revitalized in 2008 when a popular socioeconomic protest movement initiated the prior year took on political goals including secession. Public rallies in Sana'a against President SALIH - inspired by similar demonstrations in Tunisia and Egypt - slowly built momentum starting in late January 2011 fueled by complaints over high unemployment, poor economic conditions, and corruption. By the following month, some protests had resulted in violence, and the demonstrations had spread to other major cities. By March the opposition had hardened its demands and was unifying behind calls for SALIH's immediate ouster. The Gulf Cooperation Council (GCC) in late April 2011, in an attempt to mediate the crisis in Yemen, proposed an agreement in which the president would step down in exchange for immunity from prosecution. SALIH's third refusal to sign the GCC agreement in late May led to heavy street fighting, and in early June an explosion at the mosque in the presidential compound injured SALIH, who was evacuated to Saudi Arabia for treatment. June witnessed an increasing number of clashes with government and anti-government forces - some with links to al-Qa'ida in the Arabian Peninsula - in a number of cities and towns in southern Yemen. In late September, SALIH returned to Sanaa amid heavy shelling and machinegun fire between loyal regime forces and defector military forces. The UN Security Council passed Resolution 2014 in October calling on both sides to end the violence and complete a power transfer deal. In late November 2011, President SALIH signed the GCC-brokered agreement to step down, to transfer some of his powers to Vice President Abd al-Rabuh Mansur HADI, and to call for early elections in February 2012.