What was the greatest obstacle to unification of Germany?

The greatest obstacle to the unification of Germany was the rivalry between Austria and Prussia. Both countries were major powers in Europe and each sought to dominate the German-speaking world. This rivalry led to the Austro-Prussian War of 1866, which resulted in a Prussian victory and the creation of the North German Confederation. However, the South German states remained outside of the Confederation and it was not until the Franco-Prussian War of 1870-71 that Germany was finally unified under Prussian leadership.