You mean, since USA has nuclear bombs in unlimited quantity and means to deliver them on their target too, they should not have endangered life of their soldiers in Iraq and Afghanistan, should not have used minor bombs, but should have used nuclear bombs to fulfil their desire?
Sun Tzu is right though. If your goal is victory with no consideration of morality, then there are no rules in war. Obeying rules only lowers your chances of victory.
For example, the US attack on Fallujah would have been much more effective, and the American victory achieved faster, if the entire city was bombarded with chemical weapons beforehand.
In an all-out war one day, one faction may decide to disregard all rules to try and achieve a victory that is slipping out of its grasp.