According to a court, the threshold for religious accommodation is that the accommodation cannot affect unit cohesion and morale, good order and discipline, healthy and safety:
A decorated army officer is now officially permitted to wear a religious beard and turban in uniform thanks in part to a ruling late Thursday.
Capt. Simratpal Singh — a lifelong practicing Sikh, who was a West Point graduate, Army Ranger and Bronze Star recipient —was previously given a temporary accommodation in December. When it was set to expire in February, Singh sued after he was told by the Army he had to report for days of gas mask and helmet testing, citing religious discrimination.
He eventually won, with a court document saying that the temporary order would only be revoked if his beard and turban affected “unit cohesion and morale, good order and discipline, healthy and safety.”
According to The Sikh Coalition, which is currently representing Singh, he is the first Sikh American to receive a religious accommodation while serving. [Mashable]
You can read more at the link, but there have been other Sikhs who have had religious accommodations made for them by the military so I am not sure why Singh is being claimed as the first. In fact there is an entire Wikipedia page dedicated to sikhs who have served in the US military.
