Orlais didn't declare war. Or rather I don't recall the codex giving any is indication that they did. And beyond that, the Dales went far and beyond self-defense: they had a military campaign that raises a substantial portion of Orlais, with all the rape and pillage that implies. What the Orlesians did was monstrous, but everything we've seen about that period indicates the Dales were doing and would have done exactly what the Orlesians did if they had the good fortune to win the war.
According to Ages, "Orlais immediately went to war with the Dales, but was initially surprised by the ferocity of the elven response. A quick Orlesian victory was not going to happen." Admittedly, most sources say that the Chantry responded to Red Crossing, whether it's the codex entry on the Dales, the History of the Elves (from the Bioware site), or WoT. However, I don't recall any source that claims that the Dales declared war on Orlais.
Not to mention that the Emerald Knight's - the military force of the Dales (!!!) - violated Orlais sovereign border to pursue their deserter. I remember how indignant you were about the idea that Orlais sent templars into the Dales and that this was an act of war. The Dales sending a military force somehow isn't? IRL this is the kind of thing that sparks a war.
You realize there's a difference between sending armed and armored soldiers to force people to convert to a religion, and sending soldiers to retrieve or kill one of your own people who you are worried is going to reveal state secrets?
Edit: Though it does sound like you acknowledge this was a war started by both sides despite your tendency to white-wash/excuse what the Dales did, so don't take my post as too much of an attack.
I used to point out that the Dalish had their own historical account about what happened, and that we didn't know which side was correct (at the time) because of the conflicting historical accounts, and since the developers never clarified the matter at the time. That isn't a matter of "whitewashing" anything.
However, after reading the scroll from "The Knight's Tomb" and reading the response from Keeper Hawen's clan, I've repeatedly acknowledged that both sides started the war.