 After economic growth in the first two decades of the 20th century, Wales' staple industries endured a prolonged slump from the early 1920s to the late 1930s, leading to widespread unemployment and poverty in the south Wales valleys.For the first time in centuries, the population of Wales went into decline; unemployment reduced only with the production demands of the Second World War.The war saw Welsh servicemen and women fight in all the major theatres, with some 15,000 of them killed.