Attack is the best form of defense (Ukete wa Uchite). When facing an opponent there are some patterns of attack initiative but these three are the most important and principals in fighting strategies.
Sen Sen no Sen (Superior initiatory)
In this situation both you and your opponent are ready and willing to attack. Your attack must be made first in a spilt second between the time your opponent mentally commits to the attack and the moment he begins his actual movement. His commitment to attack will prevent him responding with a defense.
Sen or Sen no Sen (Initiatory)
In Sen or Sen no Sen you and your opponent begin to move simultaneously. Your awareness of his intention to attack allows you to attack just slightly faster, making your strike just before his. Also called Deai or Deashi. As a good example of Sen no Sen is Hangeki one of the five principle of shitoryu blocking system or Uke no Go Gensoku.
Go No Sen (Initiatory of defense)
You must remain calm and watch your opponent very carefully. Your block should be an automatic response to his attack and you should attack him before he is able to recover from his initial movement. In Shitoryu KarateDo, Rakka, Ryusui, Teni and Kusshin basically are Go no Sen.