A wave of anti-Armenian massacres in Azerbaijani-controlled territories started in 1918 and continued until 1920, when both Armenia and Azerbaijan joined the Soviet Union. First in September 1918, a massacre of the Armenians of Baku took place, leaving an estimated 10,000 to 30,000 ethnic Armenians dead in retaliation of the March Days . The event was later called September Days. On 5–7 June 1919 another massacre of Armenians was carried out in Khaibalikend in Nagorno-Karabakh. Up to 700 Armenians were killed in a massacre organized by Karabakh's Governor-General Khosrov bek Sultanov and led by his brother, Sultan bek Sultanov. In March 1920 a pogrom of Shusha's Armenians occurred in retaliation of the Novruz attack committed by Armenians against the local Azerbaijanis as well as the Azerbaijani army. Estimates of casualty figures are uncertain and vary from few hundred to 20,000–30,000 victims. Before and during the Russian Revolution of 1917 anti-Armenianism was the basis of Azeri nationalism, and under the Soviet regime Armenians remain the scapegoats who are responsible for state, societal and economic shortcomings. During the Soviet era, the Soviet government tried to foster a peaceful co-existence between the two ethnic groups, but many Azeris resented the high social status of Armenians in Azerbaijan, as many Armenians were esteemed as part of Azerbaijan's intelligentsia. When the atrocity-laden conflict over Nagorno-Karabakh broke out, however, the public opinion in both countries about the other hardened.