Since the incorporation of the region into the People's Republic of China, factors such as the mass state-sponsored migration of Han Chinese from the 1950s to the 1970s, government policies promoting Chinese cultural unity and punishing certain expressions of Uyghur identity, and harsh responses to separatism have contributed to tension between the Uyghurs, and state police and Han Chinese. This has taken the form of both terrorist attacks and wider public unrest such as the Baren Township riot, 1997 Ürümqi bus bombings, protests in Ghuljia, June 2009 Shaoguan Incident and the resulting July 2009 Ürümqi riots, 2011 Hotan attack, April 2014 Ürümqi attack, May 2014 Ürümqi attack, 2014 Kunming attack as well as the 2015 Aksu colliery attack. Other Uyghur organizations such as the World Uyghur Congress denounce totalitarianism, religious intolerance, and terrorism as an instrument of policy.