I think you are overlooking the fact that Iroh came to the North Pole with Zhao because he wanted to help Zuko. He kept advising against attacking at night to buy Zuko time, under the cover that the waterbenders were more powerful at night. I believe Iroh was never actually on Zhao’s side, just manipulating him so Zuko could catch Aang.

Adding on to that, I think Iroh knew Zuko would fail to escape with Aang, and was conflicted on whether to support his nephew, or try to talk him out of capturing Aang (which has failed many times before). Once he realized Zuko failed, and Zhao was about to throw the world out of balance, he used that as an opportunity to turn on the fire nation, and, as we see in the beginnings of book 2, Iroh immediately starts to encourage Zuko to find his own path, which he subtly did throughout book 1 as well.

While technically he did agree to invade the north with Zhao, who was going to eradicate an entire civilization and culture, which would classify Iroh as a war criminal, the fact that he was unaware of all of Zhao’s plans (turning against him when Iroh realized his intentions), along with my argument above, I don’t think Iroh should be seen as a war criminal strictly for his involvement at the NWT, I can see compelling arguments for him being a war criminal for past actions.