it's illegal as it was aggressive action. it was not in defence of an ally, imminent attack or an attack against the nation istself, it was not approved by the un, it was not in response to genocide. these are the only conditions in which war is legal.

declaring war doesn't happen anymore and that is accepted and does not change the legality of war. if iraq had attacked america or one if it's allies or was preparing to then the war would be legal. if there was a genocide in iraq then war would be legal. if the UN had approved the action the war would be legal. but america failed to meet any of these standards. meeting ne would have been enough but it isn't.

the nuremburg principle established that following orders was not a defence and that participating in war crimes not matter in what status is punishable. therefore american soldeirs have participated in a war crime thus incriminating themselves.