During the industrialisation of Prussia, Dortmund became a major centre for coal and steel. The town expanded into a city, with the population rising from 57,742 in 1875 to 379,950 in 1905. Sprawling residential areas like the North, East, Union and Kreuz district sprang up in less than 10 years. In 1920, Dortmund was one of the centres for resistance to the Kapp Putsch - a right military coup launched against the Social Democratic-led government. Radical workers formed a Red Army who fought the freikorps units involved in the coup. On 11 January 1923, the Occupation of the Ruhr was carried out by the invasion of French and Belgian troops into the Ruhr. The French Prime Minister Raymond Poincaré was convinced that Germany failed to comply the demands of the Treaty of Versailles. On the morning of 31 March 1923, it came to the sad culmination of this French-German confrontation.