The Preparedness Day Bombing killed ten people and hurt labor for decades . During the 1920s , the open shop efforts succeeded through a coordinated strategy called the " American Plan " . In one case , the Industrial Association of San Francisco raised over a million dollars to break the building trades strikes in 1921 that led to the collapse of the building trades unions . This employers association cut wages twice in one year , and the Metal Trades Council was defeated , losing an agreement that had been in effect since 1907 . The Seamen 's Union also suffered defeat in 1921
Preparedness Day Bombing
Preparedness Day Bombing
History of California 1900–present