Germany as we know it now started out as a great number of smaller Germanic states. Prussia was by far the largest of these. by the 1860s there was already an awful lot of water under the bridge. The states had a long history of association through the Holy Roman Empire for example, and they had common language and economic support of each other.

When Wilhelm I of Prussia came along in 1860 he put Otto Von Bismark in charge and he's really the man the pushed through the unification of the German speaking states. His efforts were hugely bolstered by industrialisation occurring at around the same time, particularly with regard to the railways. The railways fostered a natural sense of unification between the German speaking nations once they were able to easily and cheaply travel over vast distances but it also enabled Bismark's military and administrative expansion over the course of a few years.

There were various conflicts, with Russia and France getting involved at times, and trouble particularly with Austria as the Austro-Hungarian empire was still a big deal back then but ultimately Bismark's Prussia prevailed and Wilhelm I was proclaimed German Emperor in 1871.