Common Article 3 of the Geneva Conventions became effective 12 August 1949. Soon thereafter this country entered into active conflict in the Korean War (June 25, 1950 - July 27, 1953) and fought the Viet Nam War (circa 1959 - April 30, 1975), participated in invasions of small countries over two decades, and then entered into the first Gulf War (1990 - 1991).
In each of the major conflicts in which we have engaged under international law as proscribed the the Geneva Conventions we have managed to do so without further clarification of Common Article 3. It is not much of a tribute to our armed services that they have managed to comply with treaty and international law because not only is that expected of them but in fact Common Article 3 is such a simply document that understanding it requires no further guidance.
You can read Common Article 3 in its entirety at this URL:
http://www.icrc.org/ihl.nsf/WebART/375-590006?OpenDocumentAs you can see, it is clear and concise. Understanding it is not outside the grasp of a reasonable and normal literate person. Graduates of our military Academies or college Reserve Officer Training Programs or Officer's Candidate Schools should have no problem with it.
So why is a law required to clarify Common Article 3 now? I reject the proposition that our military, which has successfully complied with Common Article 3 for 57 years can not do so today. There has been no technological advance with has rendered the Article obsolete, no lowering of the qualifications of our military's leadership.