Three Hurrahs For Germany! is the decision that forms 
Germany.
It can be taken by any country with a primary culture of North German or South German.
The requirement is to own or have sphered all provinces with German cores. 
Luxembourg and 
Austria is however optional.
It is also a requirement that the forming nation has 45 prestige.
Prussia, 
North German Federation, Austria or 
South German Federation usually takes the decision.
Note that despite Ashkenazi technically is a Germanic culture, 
Israel is unable to form Germany and Ashkenazi will not be an accepted culture in Germany.