Complex sentence: The crew became involved in robbery, thefts, and, illegal gambling, but with their new partner Arnold "The Brain" Rothstein, they turned to bootlegging during Prohibition in the early 1920s.
Split into simpler sentences: The crew became involved in robbery, thefts, and illegal gambling. But with their new partner, Arnold "The Brain" Rothstein, they turned to bootlegging during Prohibition in the early 1920s.