StarCraft II - The Art of Expanding
One base usually just isn't enough. Eventually you'll have to leave the nest, and the sooner the better typically.
As if it wasn't already obvious from the above, there are a few more reasons to expand
-It's a forward base to defend.
-Having two or more income streams makes it so successful economic harassment doesn't cripple you nearly as much as when you're on one.
-People will go through your expansion before your main base, so it can give you time to produce a few more units and put up a defense at your main base if it comes down to it.
-If you have to evacuate your workers, you can send them to work at an expansion rather than having them twiddle their thumbs idle.
-More money = more production = more forces = victory.
99 percent of the time, you will want to expand to your 'natural', or your expansion right outside of your base. This enables quick reinforcement and a quick response to any threat. It also gives you better defense against harassment since your units have less ground to travel between groups of workers. From there, it's entirely up to you. Maps such as Steppes of War and Kulas Ravine have expansions behind destructible rocks that are nearby, and make for relatively simple third points. Generally your second expansion option will be far more difficult to defend than your first, so keep that in mind before you go for it.
You can be sneaky and go for a first expansion off in a corner, or at the High Yield gold minerals. The key is not to let it be known. Tech slowly and keep a mighty army of low tier units on hand to give the illusion that you're going for an early 1 base style attack. Don't send workers from your main base, if they're seen moving that direction, it may tip them off immediately that something is hidden. Play aggressively and keep them on their toes, so they're not thinking about scouting the odd spots, and only concerned with scouting your army and tech pattern. Once you have a decent economy going at the hidden expansion, throw up your natural expansion to give the illusion that it's your first expansion and that they might be ahead if they expanded before you... when in reality, they're probably way behind!
This is often times the toughest part of expanding. You want to expand when you can get away with it. Zerg players often do so very early, because when Queens hit the field, they're capable of dealing with early air quite well and once the creep expands, even Zerglings without speed can be threatening. Protoss players have a similar build order that involves a Pylon, Cybernetics Core, and Forge creating a wall that limits entry into their expansion. This only works on maps where the expansion is directly in the way of your main base, but can enable a very quick expansion, and provide a good defense point for possible rushes.
Try starting your expansion as you move out for an attack. These things take almost two minutes, and in those two minutes if you're unable to achieve victory, you can at return to a completed expansion and increase your income to come back twice as hard. What's more, your opponent certainly isn't worried about your Nexus being built, he's worried about the horde of Zealots and Sentries heading his way. Diversionary tactics often lead to the best expansion opportunities, especially at sneaky locations.
Once you you hear that initial cue of minerals being depleted, you need to start an expansion if you haven't already. By the time it finishes, a few more will have been removed and your peak income from that base will have dropped sharply--something you cannot have in a competitive game. Transfer your workers as needed and leave a few at your main base to finish off the remainder of the minerals.
Besides the obvious 'send worker, build home structure', you can increase your likelihood of a successful expansion by doing a few things
Terrans - Build your Command Center ahead of time inside your wall-in. When it's done, upgrade it to an Orbital Command, and fly it out to your natural. If you're going to be sneaky, build it in your base, load it with 5 SCVs, and send it to the island expansions or expansions behind destructible rocks. If it's scouted and you're fearful, immediately throw up some Bunkers around it. Bunkers are free, remember, so long as you Salvage them and they're not taken out. If they are taken out, you would have lost anyway without them. Basically, a wall of Bunkers can buy you time to get a real army up, or start taking advantage of that expansion far quicker than you would producing units alone in the early game.
Zerg - It is advised that before you take an expansion that might be a hot spot, that you start a highway of creep to that point so you can get your Spine Crawlers into position. Your defenses are mobile, take advantage of them! Also, you can use Nydus Worms and Networks to provide a safety net of units on demand at the expansion. However, the best defense for Zerg is a good offense. Keep the enemy in their base, and cover the map in creep and expansions while they cower.
Protoss - Start two pylons next to your Nexus at the expansion. This will give you a solid anchor to use your Warp Gates with to reinforce whenever, where ever. If you're expecting air harassment, a Pylon behind the mineral line will also serve you well to summon a line of Stalkers on demand to defend your Probes.
More games have been lost due to players not expanding, than due to players expanding. Most players also have a surplus of minerals on hand, perfect for putting down an expansion and some basic defenses. So keep these tips in mind when you're going out on the battlefield. The war will not be won without a vast income, and the person with the higher income almost always will command the mightier army!