The Steel Companies (Compania Acero) were hand-picked of the best of the anti-fascist militia of the Communist 5th Regiment.  Communists and metal workers made up the first Steel Company, but subsequent Steel Companies were organized around specially selected militia -- all extremely anti-fascist, if not communist.  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 Spanish Civil War.