Well, Orlais was weakened after the Blight. If that's not oppurtunity, I don't know what is. Of course, the Dalish bit off more than they could chew with that strategic blunder of sacking Orlais.
Umm...based on my reading the lore, the war didn't start there at all, and the Dalish weren't expanding. From what I understand, the Dalish were extremely isolationist and went out of their way to avoid interacting with humans, the Emerald Knights turning away all missionaries and traders.
Border disputes led to increased tension, which in turn boiled over when some elves sacked Red Crossing.
The Dalish say they sent away missionaries, so in response the Chantry sent templars. That does fit the Chantry's MO. I take issue with the timeline of what happened between the two events from the Dalish point of view.
The Chantry claims the sacking of Red Crossing started the war, and the Dalish were heathen who hated the Chantry. I have no doubt that elves sacked the town, but was it a rogue group, sanctioned by the Dales, or was it city elves in Red Crossing rising up after a similar event that happens in the city elf origin? That part is left quite vague.
For me, in the end, it comes down to a matter of motivation. The Dalish were active isolationists, trying to keep all contact with humans to the bare minimum, believing doing so would restore their lost
immortality (whether that exists or not is up for debate.)
Orlais was in desperate need of more land because their land was blighted. The Chantry believes that the Maker will return when the Chant is sung from all corners of the world. I personally feel the Orlesians had the most to gain from the war.
And when the elves marched on Val Reyauex, the Chantry got involved and ordered an exalted march, and I think that's when the templars came, and the Dalish ignored everything in between their turning away the missionaries and the arrival of invading templars.
Both sides have their dirty laundry, and both put all the blame on the other.