I didn't expect this to get as far as it did.
I understand that the Sikh have a tradition of martial excellence. I understand that in many ways they're very nice people, and their cultural traditions fit very nicely with APS beliefs.
I have absolutely no objection to them joining the military. Matter of fact, I'd love for them to do so. I would NOT be surprised if moderate/quiet Sikh are IN the military at this time, ones that are willing to sacrifice one of the tenants of their religion/culture in order to meet the standards of the US Armed Forces.
The military places a LARGE value on dress and appearance, especially today. There is a heavy operational bias, the current operational tempo combined with cuts in uniformed personnel means that EVERYBODY has to be deployable. Would the Sikh be willing to wear a helmet instead of a turban? Shave his hair to obtain a proper fit with a gas mask?
In comparison, providing vegetarian/kosher MREs are a minor concession, many people like those meals anyways. What does it say to everybody else if you allow somebody to totally avoid the grooming requirements, even if they keep neat and clean?
And it's not like the Military makes huge concessions to other religions - Most military members are at least occasionally asked to work during religious holidays and such.
If a Sikh, or anybody else, for any reason, isn't willing to give up that which conflicts with military needs/requirements, they shouldn't enter.