Since the 1 January 2017, Germany has changed their law, which previously clearly outlawed even preparing war of aggression. Read more and consider signing the open petition (both in German).

This is a translation of one of the comments, from a person who signed the petition:

As a member of the Bundeswehr, I am committed to maintaining peace. And the abolition of § 80 from the Criminal Code is a considerable threat to peace and, in my view, is a preparation to endanger peace in the near future.

However, already before this change, the old law didn’t stop Germany from participating in wars of aggression.