You can do a world conquest if you build up right. Basically just spam military units and then invade someone. The downside is that if you do much invading of countries you don't have a good CB on (war justification is the term this game uses) the UN might kick you out eventually, but that isn't even that bad since if you are doing a WC you eventually won't need to worry about that.
The game keeps track of your total population and reserve forces, so if you build too many buildings or build too big of a military you will run out of reserves or reach near full employment (unemployment down to like 1% or lower). Your population naturally grows over time, but the quickest way to increase population is by annexing territory.