Protoss Mid-Game

The Story:

So, you've worked past the initial rushes. You've fended off threats and hopefully some attacking of your own. But, what do you do now?

Find yourself an expansion:
Ok, so this sounds easy. There's more to it that you might think. Many protoss players that have been building up troops and defending often time don't have a spare 400 minerals for a new nexus somewhere. Do what you have to. Don't build a set of zealots and go expand. You do want to defend that expansion, right? You might want a few pylons with cannons up there, and perhaps a gateway for a small defense on-sight. Perhaps the safest thing to do is to send a larger detachment of troops to your expansion to defend. What if my main base is attacked?- you may ask. Chances are your opponett is doing the same thing you are, and his majority of troops are at his expansion.
Use new tools of war:
Although you should always be using zealots, you now have a couple new very effective units to use for either offense or defense- Reavers and High Templar.
Both of these units can take out an entire army almost single handedly. Templars with the psi storm and reavers with their splash damage scarabs.
Just a couple templar and reavers at your base can be a good defense while you pound enemy expansions with zealots. Use shuttles to carry your power units around.
Always remember the templar psi storm and energy upgrades, as well as the reaver +5 max scarab and +25 damage upgrades. They'll greatly improve their efficiency.
Be careful in control these units, as well. They're vital to your defenses, so but them behind buildings or with cannon support.
On the flip side, you can use these tools to attack while your masses of zealots and dragoons defend.
Psi storm enemy troops. Just walk right up, find a target and storm. If you don't die right away, cast against. Kill whatever you can when attacking only by psi storm. Don't expect the templar to live.
Reaver are best used while attacking by shuttles. Their slow speed will typically get them killed after getting off one scarab, but if you can surprise the enemy, the reaver will be able to get off more attacks, and could even escape on the shuttle if it can.
Battle in the skies?:
You're most likely thinking about building a stargate now. You'd love to have some powerful scouts or a few carriers for show. Although those things are great, now really isn't the time. The first air units you should be building are observers. These cheap guys can be put all over the map, and a the best scouting units in the game.
What if you're attacked by air? Wraiths (even cloaked) don't stand a chance against cannons and dragoons, not to mention the occational psi storm. And you won't really see battlecruisers at this point.
Against zerg, you best chance against mutas is still on the ground. Cannons+Dragoons+Psi Storm.
Scouts, carriers and arbiters to have their place, of course, but hurrying to get these powerful units can lead you to defeat.

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