International law vs domestic law

I don't quite understand this arguement. Although it's an international law, Geneva Convention was still approved by the Congress, am I right? So why should it has less effect than all others?

The Geneva Convention's Article 3 is "far beyond our domestic law when it comes to terrorism, and Congress can rein it in, and I think we should," said Sen. Lindsay Graham, R-S.C., assigned as a Reserve Judge to the Air Force Court of Criminal Appeals. Graham spoke on "Fox News Sunday."

I mean, with the same process, approved by the Congress, shouldn't it just be as effective as all domestic laws?