Here's an example, fresh in my mind, of an early war that just happened to me.
I start in a temperate area with forests. Research mining, start building a worker. By the time the worker is built, I've gotten Pottery researched, so i can use the worker to chop forest tiles to speed the construction of my monument and granary. Manage to get great library at turn 50. Unfortunately, I then run into Boudicca, who attempts her Pict warrior zerg.
The war lasted from turns 75-95. A worker got captured, a warrior and composite bowman were lost. Two tiles were pillaged, and maybe half my improved ones were unavailable due to besieging forces. For twenty turns i was unable to improve any tiles due to the presence of the enemy.
She lost four melee units and two ranged. She's only just reached the Medieval era on turn 142 - i'm on my 4th Medieval tech. She's now at the bottom of the demographics and another Civ has DoWed them.
Even so, i'm intensely annoyed to have lost so much time to this. I'm thinking to backstab them while they are busy with Bismark and get myself a couple of her cities - that would be some compensation, though i was after a science victory. Actually, it would be quite good if we just wipe her out.