Point of Divergence
The point of divergence in this alternate history is after the dissolution of the confederation of the Rhine in 1813 the German states decided that instead of forming the German Confederation which was a loose association of the German nations, the north German nations would instead form one singular state. The southern States do not join because of French and Austrian influences.
Creation of The Confederation
In 1815 at the congress of Vienna there is an agreement between all of the north German states except Saxony, that they would all unite to become one country known as the North German Confederation with Friedrich Wilhelm III being chosen as the monarch and Berlin as the capital. With Saxony not joining the North German Confederation it strengthens it's ties with the Austrian Empire and worsened them with it's fellow Germans.
It was decided that the government of the new nation would be a constitutional monarchy so in August 1815 the constitution of the North German Confederation is drafted.