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 seven rounds of fighting - the last ended in early 2010 with a tentative ceasefire. 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. Media reports indicated that as many as 100 protesters had been killed and many more injured amid the protests. Domestic and international efforts to mediate a resolution to the political crisis had not yielded a deal as of mid April.