This looks like a fairly legit site:
History of Circumcision
Short story: Military encouraged circumcision during World War II, making it a mandatory part of "military discipline," but it's certainly no longer a policy.
Not true that it was "military discipline". "Discipline" has no relationship to doing any sort of body modification. "Discipline" is a state of a persons mind and attitude.
NOT TRUE that it was MANDATORY in any way in any theater or in any war. period. It was sometimes offered at physicals as an elective surgery but a simple "no thanks" ended the conversations. It was NeVER a policy. But some docs being called up for mobilization from civilian life, just like the recruits, were pediatricians in civilian life & circumcision was part of their "enlightened" training of the few years previous. So now that they were in the military, they weren't pediatricians or "family docs" any more...they were general practise doing boring, repetitive medicine at bases or trauma combat medicine in combat areas. And some of the bored docs at big training bases may have offered some conscript the surgery, but it was not mandatory, compulsory, required, or even common.
I've spoken to dozens and dozens of WWII, Korea, VN vets about this. And more importantly, I've known & worked with several docs who served in those wars & they told me the truth to this rumor and all the real world background.