Just because people are allowed to use guns in the US, doesn't mean murder by them are legal.
You know just as well as me the US exploited the defense part of the UN resolutions.
The US found no evidence of WMDs, which was their so called justification to go to Iraq in defense, s the war was illegal. Even if they had found WMDs, the US use of the resolutions would be incredibly shady and worthy of bad states(and memory of NAZI Germany) at best.
You did know NAZI Germany went to Poland in defense as well?