I did the same in one game. One of the civilization was way ahead of me in term of tech. Fortunatly I had a large empire, able to produce a lot of units, and

I did the same in one game. One of the civilization was way ahead of me in term of tech. Fortunatly I had a large empire, able to produce a lot of units, and this civilization was rater small. So I sent loads of units on this civilization to take one city. Countless units died, but in the end I was able to take over the city and gain one tech. What I did later was a bit of a cheat, but it worked. I removed all my units from the city, let the other civilization retake it, reconquered it the next turn and got another new tech. I did this for a few turns, and presto, I was on par with the other civilization in term of technology. Then, it was game over for all the other civilizations as I was also the biggest producer in term of industry. As for the capital, I did the trick to make a raid on it in another game. The motivation was a little different however. They were getting really close to launching to Alpha Centauri, taking their capital destroyed their spaceship. And I was able to win. I also remember playing the game way past its end (something like year 3000 or 4000). It was quite funny, because of global warming, all the terrains were converted to swamps or deserts.