But who says all the members of the armed forces and police would follow the dictates of a tyrant like Obama? I would bet that if Obama issued such a decree, which I know he'd love to do, many people in the military would think about staging a coup. Certainly many, maybe the majority of military people side with 2nd amendment gun owners. In the case of civil war, you'd most likely have the great majority of armed citizens and the majority of the military on the same side. Many big city police department members would side with Obama, but most small town and country police would probably be with us as well.
History. In every dictatorship in recorded history there has never been a shortage of wannbe thugs, concentration camp guards, throat cutters (Cambodia) who were ready willing and able to arrest, brutalize, and murder their fellow citizens at their governments' behest. SOME cops might quit, but the majority didn't join the police to question orders or to judge the constitutionality of laws. As one self identified cop on FR told me - if you have a problem with the law, take it up with the legislature because it's my job to enforce it not question it.
Oh and BTW in the NYC gun confiscations in the early '90s there was not a single reported instance of cops failing to arrest anyone who had one of the evil "assault rifles" after the confiscation deadline had passed.