According to Dollard, frustration always leads to aggression. This hypothesis is based on the psychodynamic concept of catharsis.
When attempts to reach a goal is blocked by external factors, this leads to us experiencing frustration. This frustration leads to a buildup of our aggressive drive that (at some stage) leads to aggressive behaviour such as physical violence.
The release of the aggressive drive created by our frustration is cathartic as it's now satisfied, and makes us feel better as we have 'gotten it off our chest'.
Catharsis reduces the aggressive drive and means that future aggression isn't likely.
Dollard also recognises that aggression isn't always directly expressed towards the source of aggression. There are three reasons for this:
It's too abstract and not physical. (eg. the economy)
It's too powerful and there's risk of punishment (eg. a teacher)
cause of frustration isn't available at the time (eg. the person is far away)
In cases where the frustration can't be directly expressed onto the source, the aggression is displaced to an alternate target, one that isn't abstract, is weaker and is available (eg. an inanimate object)