Most of the coal mines in Britain were purchased by the government in 1947 and put under the control of the National Coal Board, with only the smaller mines left in private ownership. The NUM had campaigned for nationalisation for decades and, once it was achieved, sought to work with the NCB in managing the industry and discouraged strikes. Under the chairmanship of Alf Robens, pit closures became widespread as coal's place in energy generation declined. The NUM leadership continued to resist calls for strike action, but an unofficial strike began in 1969 after a conference pledge on the hours of surface-workers was not acted upon. This was a watershed moment that led to increased spending on the coal industry and a much slower rate of pit closures, as well as the election of more militant officials to the NUM leadership. Under the government of Ted Heath, an official strike in 1972 won increased wages after the Wilberforce Commission. Less than two years later, Heath called a general election over another official strike, called after an overtime ban had led to a Three Day Week in Britain, and lost the election to the Labour Party. The wage demands were then met and spending on the industry continued to increase, including the establishment of the new Selby Coalfield.