If you can't supply one the logical conclusion is that there is no good reason and all the reasons that flow from simple observation are terribly unsupportable morally and legally. You know that. You are vague because giving a better answer is impossible and the actual answer is unpalatable.
Yet the reason is an important subject, and in questions like that different reasoning lead to lost lives. So it's a serious, justifiable subject to discuss.
Everyone bombs everyone, it seems. NATO bombed Serbia, Afghanistan, Iraq,... Sent "peacekeepers" to loads of African countries (funny enough those countries sit on tons of raw materials). Russia bombed Georgia (not the US state of Georgia), Ukraine. China isn't bombing as much with actual bombs for now, money seems to work just fine it seems. Saudi Arabia is bombing Yemen, Israel is bombing Palestine, while Palestine is bombing Israel.
No need to justify any of that, because it all sucks and is ultimately utterly pointless. It does help so, to understand the other side. Because it makes it easier to find a solution that doesn't lead to all out war between nuclear powers. And no, I don't consider just handing Ukraine to Putin to be that solution. Or Taiwan to China, as far as that is concerned.
NATO did not go to war in Iraq as an organization. That was done by an independent coalition. After Saddam's fall, they did however take part in training the new Iraqi military.
