Generally speaking, "assault" occurs when someone threatens bodily harm to another in a convincing way. Assault often is followed by battery, which is defined as unlawful physical conduct (often an act of violence, but also unwelcome sexual contact). Not all threats are considered assault.
The fight-or-flight response plays a critical role we deal with anger, stress, or danger. It prepares our body to fight or flee from the threat. Without it, the body cannot help you protect yourself in a dangerous situation. We lose much needed energy and strength. We cannot really defend ourselves!