However, as I traversed the countryside, a dragon started attacking three giants in the distance. I decided to get up close and see what happened. The giants killed the dragon with ease. But then, even though there were at least two hundred meters between me and giants, the giants started attacking me!
I tried to stand against them toe to toe (my character used a blade), but I lost most of my health and decided to run away, back to Whiterun. On the road back to Whiterun, I saw a lone hunter and his dog attempting to come to my aid. They were killed pretty quickly. Then, as I neared an Imperial fort, two soldiers on patrol tried to help me, but they were dispatched quickly, too.
I retreated into the fort and got out my bow. The giants couldn't reach me nor the archers on the battlements, and it was only then that the three giants were finally killed. Casualties: Giants: 3, Imperial Soldiers: 8, Misc characters: 1.
The dynamic nature of these events are what make Skyrim so great.
And beware... even at level 22, giants are really dangerous. I can barely kill one all by myself.