Currently on my second attempt to get into DF, still in my (re)training fortress using a newby guide.
My fort was going superbly, everyone happy and well-fed, with around 40 population. But I was dicking around so much with custom stockpiles and engraving my dining room and making the yet-to-be-needed Noble quarters look nice, that I neglected to make a military at all.
I knew a military was a requirement but I thought I'd have more time for some reason.
A goblin ambush waltzed up to the undefended and wide-open fortress entrance. With the game on Pause I made my first military squads of about 8 dwarves in total, completely unarmed, and sent them up to intercept. There was a bloody encounter just before the stairs leading down to the main fortress, and my impromptu military was victorious!
All but one of them was killed or seriously injured, though. There was a minor tantrum spiral that led to me having to lock about 5 other dwarves in their rooms until they died.
I was just getting things back together, when there was another goblin ambush.
Again, I thought I'd have more time to prepare for the next one, so I
still didn't have a proper military.
I sent another hastily assembled squad of unarmed wrestlers to meet the invaders. Many goblins got all the way to the underground levels this time.
A massive wave of immigrants had arrived just before this second attack.
Total chaos. It seemed to take ages for all the goblins to be eliminated. I still have no idea exactly how it all went down. At one point I just gave up trying to manage the disaster and watched.
A MAJOR tantrum spiral ensued. There was blood and dead dwarves and goblins everywhere. I locked about 10 dwarves in their rooms. Lots of red-arrowed dwarves milled about without ever going back to their rooms.
I built a load of coffins and had to expand the graveyard. All of these coffins contain a body (and there were still several bodies laying around at the time of the screenshot, as well as some live dwarves still locked in their rooms):
I lost all my legendary miners, including the expedition leader who went berserk in the dining room and splattered my lovingly engraved walls and floor with the blood of two dwarves before being taken down.
Things have settled now. Somehow, the fortress never fell but it was close. And, of course, a lot of Fun.