The United States continued to retain its global superpower status while China, along with launching vast economic initiatives and military reforms, sought to expand its influence in the South China Sea and in Africa, solidifying its position as an emerging global superpower; global competition between China and the U.S. coalesced into a "containment" effort and a trade war. Elsewhere in Asia, the Koreas improved their relations after a prolonged crisis and the War on Terror continued as Osama bin Laden was assassinated by U.S. forces in a raid on his compound in Pakistan as a part of the U.S.'s continued military involvement in many parts of the world. The rise of the Islamic State of Iraq and the Levant extremist organisation in 2014 erased the borders between Syria and Iraq, resulting in a multinational intervention that also saw the demise of its leader. In Africa, South Sudan broke away from Sudan, and mass protests and various coups d'état saw longtime strongmen deposed. In the U.S., celebrity businessman Donald Trump was elected president amid an international wave of populism and neo-nationalism. The European Union experienced a migrant crisis in the middle of the decade and the historic United Kingdom EU membership referendum followed by withdrawal negotiations during its later years. Russia attempted to assert itself in international affairs, annexing Crimea in 2014.