How we defined success in both WWI & WWII was our (the US) adherence to a higher moral code than the cynical realpolitik of the continental powers. This was expressed with Wilson's proposal of the League of Nations, and the Geneva convention. Even in our worst lapses of WWII (the Japanese internment camps), we never allowed ourselves to sink to the levels that we sank to in Iraq.
The abandonment of human rights principles- in Abu Ghraib, in Guantanamo, in the legal counsel Bush sought to allow torture- has been by far the most significant casualty of this war, and on those grounds alone I consider this war a failure. If the US doesn't stand for the basic human rights outlined in the Declaration of Independence and the Constitution, then it doesn't stand for much at all.