starcraft ii walkthrough

The race of the opponent also gives you an idea of likely tactics. With Zergs, you can expect a massive early rush of Zerglings as their first offensive Starcraft 2 strategy that can effectively cripple you by killing your workers and destabilizing your economy. They are particularly effective with the Speed upgrade. Zergs also go for early game Roach rushes which are also quite deadly. Adequate scouting will give you warning signs and you can prepare your own Starcraft 2 strategy accordingly.
Source: starcraft 2 campaign walkthrough