Oh, boy is it ********... Here's a prime example:
I was testing my Final Fantasy mod, and I'm playing as the Dwarves (Ind. Sci. and Mil)... I'm in the late industrial age (Machina Warfare is due in 1 or 2 turns), and cover about 2/3s of the continent (which is fairly large: I have about 1/3 of the entire planet under my control), have a HUGE army (although it's "average" compared to my next biggest rival) made up of the best units available (I upgraded all my stuff by doing a few turns at min. science), with rail links to every city in my empire.....
The other guy on the continent is the Kingdom of Baron (Mil, Rel, Exp), which has JUST enetered the industrial era, and the only industrial tech they have so far is Nationalism (I saw at least 1 Gunner in their cities), and I'm selling them some resources (1 strat and a few lux), and suddenly, without warning, they attacked one of my southernmost cities (which was squeezed in between two of thiers).
They exhaust their entire arsenal of good units (Mages, for the most part, which have high enough attack to be a serious threat to even modern units, but low enough def that even medieval units can kill them fairly easily) tryin to take the city, and only manage to reduce the city to about half its defending force (I've learned early in in Monarch games to garrison one's border cities with the defensive equivalent of a Stack of Doom due to the AI constanly pulling crap like this!).
On MY turn, I unleash my horde of Steam Tanks, Steam Airships, Mages (all my cities but maybe 2 or 3 have Mage towers, which generate mages every 15 turns), Dwarf Marines (my best attackers at this point), and proceed to roll over 8 of his cities in 1 turn.... As it stands now, I could easily conquer his entire kingdom in about 4 turns (especially since I've kicked all my higher production cities on to cranking out Steam Airships and Tanks)
Basically what I'm saying is that any human player would have been smart enough to realize they didn't stand a chance under those circumstances, and would have just chilled.....