Obama said he would close the military prison at Guantanamo.
As commander in chief he has all the necessary powers to do so.
He can order the military personnel there to lock all the doors, leave.
Prison. Closed. Done.
Arguments about the prisoners contained within are moot.
If congress wishes to block the removal of the prisoners, then it is no fault of the president's if closing the base causes them harm.
It should be up to congress to decide whether or not to intervene in order to save their lives.
If they wish to control the prison, then congress should take responsibility to assign proper civilian oversight.
In this case it would no longer be a military prison. Still, closed.