Insubordination.
He refused a direct order, that comes under the direct authority of the Commander In Chief of the military. As to the authority and legitmacy of our current President - this was an attempt to force that hand.
However, Pres. Obama is not on trial (yet). Pres. Obama has not broken the law (or been charged with it), yet.
This man actively and with conviction disobeyed a direct command, and a lawful order from a superior officer. This is not without consequence.
If it turns out that Obama is not valid will the verdict be overturned ?