Regarding AI opponents:
Quantity of AI opponents is a major factor. If you're playing against 2 or 3, with superior tactics you can wear them down and then start moving into their territory and taking over. But if you're playing against 6 or 9, if you demolish an AI opponent, you find one of his rivals moves in. Pound him, and the first has grown in strength to take over. Actually expanding is *very* difficult; you will find yourself spending most of your time fending off his offenses.
Focus your efforts on preserving your forces while demolishing his; don't waste troops trying to defend territory when a larger AI force shows up. Retreat, smash it when your forces have concentrated, then move back.
BTW, if you're playing the demo, you're at a huge handicap. You can't go thugging with research 4
