Eritrea operates under two key geopolitical constraints: multiple security concerns and a weak economy caused by the country's small size and population (5.2 million in 2011, according to the United Nations) and lack of natural resources. The war-torn country has few options for satisfying its economic needs. Since gaining independence from Ethiopia in 1991, Eritrea has fought wars against Yemen, Djibouti and Ethiopia. The war with Yemen was sparked by territorial claims over Greater Hanish Island in the Red Sea. In its conflict with Djibouti, Eritrea claimed ownership of the Ras Doumeira mountains along the coast. Eritrea has faced an existential security threat since independence: invasion by Ethiopia. This concern grew after the United States allied with Ethiopia and diplomatic quarrelling froze relations between Washington and Asmara in 2001.