CalPundit provides a great analysis and documentation of President Bush's military record. "To make a long story short, Bush apparently blew off drills beginning in May 1972, failed to show up for his physical, and was then grounded and transferred to ARF as a disciplinary measure... Bush's record shows three years of service, followed by a fourth year in which he accumulated only a dismal 22 days of active service, followed by no service at all in his fifth and sixth years. This is because ARF duty isn't counted as official duty by the Texas guard."
"So Bush may indeed have 'fulfilled his obligation,' as he says, but only because he had essentially been relieved of any further obligation after his transfer to ARF."