On September 18, 2001, Congress passed the Authorization for the Use of Military Force (“AUMF”) authorizing the president to prosecute the "war on terror."
The Bush Administration held that the President, as Commander-in-Chief, could hold “detainees” until the “war on terror” was over, without reference to the Geneva Conventions.
ADDITIONAL RESOURCES ABOUT PRESIDENTIAL POWER
Quick General Reference on Presidential Power
Wikipedia article on "Powers of the President of the United States"
Presidential Power: Further Reading
The Imperial Presidency by Arthur Schlesinger, Jr., is the essential post-Watergate survey of the waxing and waning of theories of presidential power in the United States since the nation's founding.