Once you get tanks, yes, oh boy can you do lightning campaigns. In fact thats the only way to wage war in democrcy, I believe, since the war weariness is incredible, if the war goes on for too long.
In the game I played I had two or three of those. I was playing on the easiest level as the romans, but anyway, here goes. First I built up mass of culture, with a load of wonders and temples. Then I turned on my weakest "best friend" the egyptians, who was "in awe" of my culture. And I ganged up with basically all my other "friends" the iroquios, the greeks, russians and english. The egyptians were toast. While my allies did get a few cities too, they were soon assimililated into my culture once the war was over. This war took place in the middle ages, with basically legions and knights. I had one catapult which was hardly used.
Second time around, was when I decided it was time to backstab my "eternal" friends the Iroquios. I produced large masses of cavalry, infantry and marines, and swarmed all in over the borders, still keeping peace with the greeks on the same continent. The war didn't go as well as planned, so I settled for peace, and demanded an Iroquios city in return, which they obliged to.
Then I got a rubber resource from somewhere in the new acquired territory and in the meantime obtained the tech to produce tanks. And then, with 3-4 high-productive cities producing tanks (3-4 turns), I declared war on greece, who was falling behind. I rolled in with the tanks. In basically 4-6 turns the greeks were toast and lost their remaining 4 cities, and the Iroquios were finished pretty soon after that, with the same force that took greece. Marines and infantry were sent in after the tanks to keep things quiet and settle discontent among the population.
The downside from this, was that the english and russians declared war on me for sneak attacking greece, and started bombarding tile improvements all over the continent. They also managed to place/conquer a few cities. But very quickly the were thrown out by the tanks, and the marines were cleaning up.
After I cleaned up the continent, I simply made peace with the english and the russians, and produced a s***load of workers to repair tile improvements all over the place. It all took about 10-15 turns, and it was in the near-end game around 2000 A.D. Basically that lightning campaign won me the game. The tanks simply rock, especially against a weaker opponent.
Only a few times did I resort to resolved bombardments of cities. Both were in places where the terrain made it profitable, and I could do it at a somewhat safe distance. But I really appreciate the lightning campaign strategy. -BUT I s'pose you really cannot do anything like this, if you don't have the culture to support it, or if you're not resolved and strong enough to finish it. Otherwise the conquered cities will still be able to defect to the Vaterland.
Once you get around 6 cavalry most anything is possible against a weaker Civ. I was averaging two cities per turn against the English.
