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Let's say you were just attacked, and you repelled your opponent's troops. That's great! Now, if you still have standing forces, this is the best time to retaliate. If you managed to completely destroy the opposing force, the upper hand is yours for the time being. The faster you make your move, the less time your opponent will have to recover. Send in a scout ahead of your main force to see if your opponent has a reserve army or lots of static defenses at his base. Also, be careful when fighting zerg or protoss because they can replenish their numbers in the blink of an eye, or they might have burrowed troops lying in wait.
Sometimes your opponent will launch a massive attack on your base, one you know you can't repel with your current army. In such scenario, it could be a good idea to avoid their troops and attack their base instead. Your opponent may call off the attack and rush back to defend his base. If he does not, your army will be doing more damage this way than it would in a lopsided battle. There's always the possibility to outlast a foe and achieve victory if you manage to destroy their economy and you're able to maintain and defend an expansion (or two).
- Map control involves vigilance and heavy firepower to keep the enemy in their base. Deny any expansion attempts and their resources will eventually dwindle. This will also allow you to expand in relative safety (watch out for drops and sneak attacks) while you build up your forces to deliver the killing blow. Map control can also mean controlling the vision of the map by holding Xel'naga watchtowers with zerglings, marines, hellions, or stalkers. The essence of map control is denying your opponent's vision around the map.
- You can build base defenses outside their base or set up a blockade with your army.
- Scouting is crucial to maintain control of the map. In the early stages of the match use workers, overlords, zerglings, marines or even zealots (though these are pricey to use as simple scouts) to keep watch on the map. Later on you'll have access to scanner sweeps, overseers, observers, and air units.
- Watch out for terran command centers flying out towards new resource spots. Check for nydus wormsign when fighting zerg, and be on the prowl for flying transports. Try to keep one unit in every expansion spot. You never know when a desperate opponent might try to go for all the marbles, but you want to know about it when he does.