Your civ must have already been at war with them, or you cut down ALOT of trees.
...actually, it's possible the game figures out how many trees you've cut down by how many logs you've created. Which, now that chopping down a tree yields a ridiculous amount of logs, could equal instant war with the elves. Hehehehe.
Oh no, was at peace with them the second time the caravan showed up, and summer rolls around the same year and the civ screen switched to 'At War' and I got reports of an advancing elven army, two fortresses being conquered, and my king dying without an heir (in fact, the liaison's oldest daughter took over.) The implications are the capital has fallen and the king was killed in battle. Also, it's obvious this war was premeditated and probably in the making on the elves' part with the rapidness of thier advance in just a few short months.
So now Spiritnets (my fortress which was a reclaimed worldgen site that had been trashed by a titan,) is preparing its populous for war. Copper, iron, steel and industry. It will meet wood, claw, and nature on the field of battle in the Hinterlands of The Special Diamond. As an act of war/show of force, all the trees in our surface holding have been felled and many smaller plants ordered to be torn up by herbalists.
Nature will find no place here.
@Splint:
Scent may definitely be a factor. I turned the thing's odor value thingy to like 1000 (I figure that a being made entirely of assorted putrified organic matter probably don't smell like a spring breeze).
That's my assumption since dead humans (I got attacked by a bandit gang in summer of year 2 in Spiritnets,) caused several dwarves to be overwhelmed by horror, presumably from the stench of death. Also the first kill of the fort was a human my once-massive-coward militia commander punched. Hard enough to cave his chest in. Which then actually horrified both her own soldiers and several bandits.