Political debates after the transfer of sovereignty have centred around the region's democratic development and the central government's adherence to the "one country, two systems" principle. After reversal of the last colonial era Legislative Council democratic reforms following the handover, the regional government unsuccessfully attempted to enact national security legislation pursuant to Article 23 of the Basic Law. The central government decision to implement nominee pre-screening before allowing Chief Executive elections triggered a series of protests in 2014 which became known as the Umbrella Revolution. Discrepancies in the electoral registry and disqualification of elected legislators after the 2016 Legislative Council elections and enforcement of national law in the West Kowloon high-speed railway station raised further concerns about the region's autonomy. In June 2019, mass protests erupted in response to a proposed extradition amendment bill permitting extradition of fugitives to Taiwan, while protesters argued that criminals might be extradited to mainland China. The protests are the largest in Hong Kong history, with organisers claiming to have attracted more than three million Hong Kong residents.