[MosesKnows]

You confuse impetus with a cause.

You could very well be correct considering that impetus is a noun which means that which causes a given response.

[TopQuark]

To clarify: that which immediately precedes the event is not necessarily its cause.

E.g.: the infamous assasination is not the cause of WWI, for instance.

Next time, check the dictionary:

n. im·pe·tus

1. An impelling force; an impulse.
2. The force or energy associated with a moving body.
3.  
   1. Something that incites; a stimulus.
   2. Increased activity in response to a stimulus: The approaching deadline gave impetus to the investigation.