 The latter became much more profitable after the introduction of customs controls in 1900 and especially with the end of the 50-verst free trade zone along the border.The Qing authorities unsuccessfully tried to encourage Han farmers to settle there but from 1905 they replaced indigenous officials with Han men, much to the chagrin of the Mongols.After the revolutionary turmoil of 1911, China struggled to reassert control of the Hulunbuir area which was partially achieved in 1915, fully only in 1920.