around the beginning of the 20th century, there was a religious conservative resurgence in america. it's known as the temperance movement. they tried to push back against pleasures of the flesh and wanted to cut down on sex, drugs, and jazz music (rock and roll didn't exist yet). that was the reason for the american alcohol prohibition from 1920 until 1933. it was also the reason americans started circumcising. the belief was that if you cut off the most sensitive parts of a man's penis so he'd get less pleasure from sex, he'd be less sex focused and less likely to engage in sinful behavior like premarital sex, adultery, and masturbation.

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there was also the belief that men literally lost energy through the loss of semen, so sexual excess was seen to literally be responsible for poor health and numerous diseases. in that sense, chastity was thought not just to be good for the character and the soul, but for overall health in general. if you've ever seen Doctor Strangelove, that's the mindset the jack d. ripper character was satirizing.

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here's a collection of numerous citations from peer-reviewed medical journals where dozens and dozens of american doctors gave that rationale for promoting and performing circumcision:

http://www.drmomma.org/2007/05/circumcision-brief-history-in.html