A long and bitter strike by coalminers protesting about unemployment and low wages collapsed in 1926 amid terrible anger and distress. However it was not only coal which declined but also other heavy industries which had been the mainstay of the Welsh economy since the Industrial Revolution. Workers in steel, tinplate, slate and transport were badly affected. In west Wales the woollen industry rapidly disappeared, mill after mill falling derelict in the Teifi valley. By 1900 copper smelting had reached its peak and was defeated by competition from abroad, but Swansea, its home town, had meanwhile grown into a significant port, second only to Cardiff and ready to accommodate new industries like nickel production. The largest ironworks in the world, Cyfarthfa in Merthyr Tydfil, closed down in 1921.

After the Second World War, sometimes as a result of slum clearances, there was an urgent need for more housing and council house construction, a new form of heavy industry, resulted in large estates in Swansea, Newport and Cardiff.