This may be a 'facepalm' moment.
Haven't played DF in a while, picked up 31.19 to get back into it. Load my usual setup, modified to include a doctor. Mine out a space to put my food stockpiles in so they don't rot, then start putzing around with other stuff, but no military or defense. Went on like this for about a year, and then the dwarves started hunting vermin. Oops, forgot to set up farming. Do a quick farm set-up using floor hatches from above and a drainage ditch, get farming working, get my dwarves up to like 1000 food, 1000 booze after another year or two. Notice I still haven't bothered to do military. I say to myself, "Self, you haven't set up a military...but you also haven't seen any snatchers or thieves. Well, whatever." Self still does not bother setting up military.
And then the seige comes.
I draft 21 random dwarves (since I still don't quite get the DF2010 military system, despite reading the wiki), send them against the seige, and hope that it's enough. Well, 4 dwarves die, 6 go to hospital and eventually recover, and the seige is broken. Hot damn, I say! And then I return to my normally scheduled ignoring of the military system. Break through to the first cavern, pit a llama baby to do some exploring for me (he's wandering around with red injuries coz pitting him led to a 9 z-layer fall). Start doing the dining room and offices for nobles, and...AMBUSH...I say to myself again, "Self, this time, let's figure out what we're doing!" So, I pause the game, use Dwarf Therapist to figure out who has essential skills and who can go in the military, set up custom professions, then assign them all to squads, then assign the squads to go kill the goblins. You can predict what happens next. I sent 3 squads of 9 out to meet the goblins. They came back with 1 or 2 in each squad...or should I say, those guys were probably eating and didn't even go out. Tantrums started erupting. I hadn't set up burrows to keep the civvies indoors, so they were all pathing out to get the dead bodies and dropped crap, and getting killed by goblins.
Another fortress killed by overconfidence and laziness.
I don't like how rare metals are in 31.19. I had plenty of wood to smelt metal and make stuff out of it, but no ore to do so with. Oh, I could have bought it, but where's the fun in that?
Anyway, I'll try again, I'm sure. And this time, I'll build farms and a military infrastructure much sooner.