The Geneva Conventions evolved in the course of the 20th century in response to the massive global armed conflicts of World Wars I and II. Along with the evolution of the Uniform Code of Military Justice (UCMJ) in the United States, the Geneva Conventions recognized that even in the context of war and the military, rules and process apply.
For the first two years of the “war on terror” (2002-04), the Bush Administration’s position was that it was not required under international law to provide any legal process to detainees.
The Administration held that because the suspects were not part of an organized and uniformed national military force, they were not entitled to the protections of the Geneva Conventions. In effect, the Bush Administration held that the President, as Commander-in-Chief, could hold these "unlawful enemy combatants" until the “war on terror” was over.
ADDITIONAL RESOURCES ABOUT THE GENEVA CONVENTIONS
Quick General Reference on the Geneva Conventions
Wikipedia article on "Geneva Conventions"
Geneva Conventions: Further Reading
Basic rules of the Geneva Conventions and their Additional Protocols