When did it become common for non-jewish parents in america to circumcise their boys?

Common for >90yr: The move to circumcision in this country predates any practicing physicians i know of. In some populations it became a proof that your family could afford a physician assisted delivery & care & may have persisted out of habit. Wide spread tonsillectomy and tongue clipping have come & gone but personal preference has kept circ rates high.

World War 2: General mcarthur required all troops headed for asia to be circumcised because of foreskin related problems in the tropics, jungles etc., when men could not wash, were wading rivers etc. Nationwide campaign was instituted promoting all the benefits of circumcised penises being cleaner, healthier and avoidance of infection, cancer, STDs etc.. Then circ'd fathers wanted same for their sons and so on.