Instructions in more recent resolutions replace instructions in older resolutions, Teribus. The resolutions that instruct the US to honor its commitment to the sovereignty, territorial integrity and political independence of Iraq are more recent than the one that authorized member nations to use all means necessary (etc.).

Therefore, the instruction to the US to honor its commitment to the sovereignty, territorial integrity and political independence of Iraq is the one that the US was obligated to obey. This means that not only did the US not have authorization from the UN to do what it did to Iraq in 2003, but it means that the US had explicit instructions from the UN to not do what it did to Iraq in 2003.