I'm still really new to the game, which was apparent when I dug into a cavern. I set up a defensive line in the tunnel leading to the cavern, figuring I could seal it off if I ran into trouble. I was mining out some gems when I got attacked by a blind cave ogre and it's buddy. The ogre killed a fisherdwarf, so I figured 'an eye for an eye'. these two ogres proceeded to beat my military to death. I try calling them back, but they keep getting interrupted by the ogres, so I go to seal the entrance to the cave and cut my losses.
The problem is, I was so busy micromanaging the miner and fisherdwarves (and my mechanic who kept hauling when I turned it off) in the cave that I forgot to check up on the constructions, which were never built. I forgot to designate a new mechanic after my other one got trapped in the cavern. The ogre's proceed to wander into my bedroom area and kill every dwarf within sight (one of whom ended up with a full title to her name). The last Dwarf standing is my manager. The titled Ogre chases him down into the cavern, beats him into submission and walks away, leaving my noble for dead. He lays there, comatose for a full winter and half of spring, until a migrant wave arrives.
The migrant wave has some fairly skilled combat dwarves (primarily 2 marksdwarves and an axedwarf). They are immediately drafted to wipe out the two ogres, who seem to have moved into my fortress. They charge against the titled ogre, who puts up strong opposition at first. Finally a good shot lodges into her heart and the battle turns, eventually felling the bitch. The other ogre turns out to be a challenge for the military, but they're holding an even battle and the ogre is still fairly tired from his last rampage. Suddenly a flock of Elk Birds arrives from the caverns, flanking my military and saving the ogre (I did nothing to antagonize these birds). My military falls before I have a chance to send in backup.
I rage, and seal the bedroom floor, circumventing the bedrooms and abandoning the area and items there rather than lose more dwarves. Life continues on, with some very miserable dwarves. Cue tantrum spiral. The migrant wave gave me 24 new dwarves, and the battle killed 4, leaving me with 20 working dwarves and one ex-manager who somehow wouldn't die. Tantrum spiral kills all but 8 dwarves, leaving me to reassign jobs so the Fortress can sustain itself until the next migrant wave. Just when I think I'm ok, I accidentally break ground into a new cavern, pissing off a cave spider who decides these dwarves must be sacrificed. It webs every dwarf I have left, bringing life to a standstill. It then systematically tears apart each dwarf piece by piece until there is nothing but partial corpses, body parts, and enough dwarven blood to sate Armok for the day. All because I forgot to assign a new mechanic.