The Steel Companies (Compania Acero) were hand-picked of the best of the anti-fascist militia of the 5th Regiment.  Communists and metal workers made up the first Steel Company, but subsequent Steel Companies were organized around specially selected militia.  There were stringent requirements for getting into a Steel Company, and the morale and sense of purpose was very high.*
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Steel Companies had special slogans designed to create an iron unity, and two of the most important were:*
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Never leave a comrade, wounded or dead, in the hands of the enemy.* 
If my comrade advances or retreats without orders, I have the right to shoot him.*
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28 Steel Companies were formed during the Civil War.