Complex sentence: The strike, which lasted 11 months, produced no gains and demoralized the miners, was fought vigorously with strikebreakers and reduced steel production which maintained coal inventory for sale to the company's customers, thus preventing state interference due to a shortage of coal.
Split into simpler sentences: The strike, which lasted 11 months, produced no gains and demoralized the miners, was fought vigorously with strikebreakers and reduced steel production. The company was able to maintain coal inventory for sale to the company's customers, thus preventing state interference due to a shortage of coal.