As for this particular thing, you have to really, really try to get the elves to attack. And then you have to try harder. Additionally, it is generally considered beneficial to piss the elves off because that results in them bringing less high value per weight items and more low value per weight items -- which sounds bad, except the "high value per weight" means cloth and "low value per weight" means tamed exotic animals. Get them angry enough and they might just bring a breeding pair of giant eagles and then you'll be sorry! They'll show you, you mongrel treecutter you!
Myth: Elves consider cloth to be worth more per weight and thus bring more of it when they're happy. This is
false.
Fact: When caravans load their pack animals and wagons,
items in bins are loaded first (technically, they're loaded immediately after any goods you requested from the liaison or merchant noble), and trade goods are mostly distributed evenly throughout the various categories (metal bars, crafts, cages/animals, buckets, clothes, weapons, armor, cloth, leather, booze/barrels, extracts, meat, plants, cheese, etc.); thus, if a very happy civilization decides to bring so many trade goods that the allotted cloth and leather bins are enough to fully encumber their animals and/or wagons, then that's all they'll bring. This was notoriously easy to do in 40d with the Elves because they didn't bring wagons, and 50+ bins of cloth was more than enough to fully encumber all of their pack animals - if you allowed said animals to carry more (which is pretty much how it's working in 0.31), you'd get a
vast amount of trade goods covering the full assortment of types.