To elaborate on the above - civs with the BABYSNATCHER and/or ITEM_THIEF tags will always be hostile to everyone else (including any civs they'd otherwise get along fine with) throughout worldgen, and will always be hostile to you in Fortress mode. This is not the same thing as two civs being at war - it'll get you the same relations as vanilla Dwarfs -> Goblins - there are a few differences in the way wars work, mainly during worldgen/legends, but nothing overly significant.
Modding ethics is the best way of getting actual wars to occur, but it's a very muddy process because there are certain factors which have an immense factor as opposed to other factors - Toady made a post about it a long while back somewhere which basically explained the effect each ethic has on potential relations. And if you're anything like me you won't want to sacrifice the flavour you can get out of modifying civ ethics just to slightly increase the chance that two very specific civs will start fighting - and even then they'll only do so if they actually meet each other and are in the same area.
Oh, also, in case you ask: There isn't any real way of modifying the size of sieges accurately.
I'm sure I've forgotten something... oh yeah, the chances of a civ going to war are influenced by the personality of its leader. So aggressive or generally warlike civvers will get into wars more often, while more measured ones won't, and such.