Remember, first and foremost, that although purchase units, combat move, combat, and noncombat are nominally different steps, they are really very connected. If you anticipate that your combat moves, combat results, and noncombat moves are going to, say, result in a severe lack of force in the east in the next three turns, you should probably build some infantry to march east to make up for that lack. (Or you may decide to abandon the east).