I've never been good at early military building or expansion.
BUt my current (only) game has me in a late industrial time, with only 7 cities (big cities, but only seven). I was running a democracy with many many turns of peace. One of my cities didn't even have a single military unit in it for several hundred years.
Well, I decided to pick a fight, and since most improvements were built, I just started cranking out tanks in every single (7) city. Occassionally rush building.
After 10 or so turns I had an army a billion times bigger than previously. Then just for practice, I initiated a worldwide war. While my force of 10 tanks could put a hurt on whatever it attacked, I was spread too thin, and it was really really hard to take a big city with a bunch of infantry in it.
So, I decided I needed evern more units next time.
So, moral is: if you dedicate lots of cities to building forces over a medium-long stretch of turns, you can do it.
ER