Officers in the U.S. Army Reserve are learning about the small print in their commissions. Harkening back to the ancient tradition that an officer was an officer for life, the army is invoking a regulation that allows an officers "offer" to quit ("resign their commission") to be rejected. This is done when an officer has a critical skill that is in short supply, or is due for a trip to a combat zone, and replacements are hard to come by. This policy has been used since 2003, and about four hundred officers have, so far, been told to keep their commissions, at least for the moment. Ten of those officers are currently suing the army.
The whole officer thing descended from the ancient concept of "royal officers." Knights were royal officers, and once you were one, you were one for life. Several centuries ago, the modern concept of officers evolved. But these officials could be inactive (because they were not needed at the moment) or retired (due to age or injury) at half (or no) pay. Since, originally, you became an officer by invitation ("commission") of the king, you couldn't just walk away without offending His Highness. The State has replaced The King, but the relationship has remained, at least in the small print, remarkably stable.