Final answer:
The constitution for the German Empire was not successfully agreed upon in 1848 as intended by the Frankfurt Assembly. German unification and the establishment of a constitution occurred later, culminating with the proclamation of the German Empire in 1871.
Step-by-step explanation:
The constitution for a united Germany was agreed upon during the Frankfurt Assembly in 1848. However, this attempt at unification and constitution-making was not successful. Notably, Friedrich Wilhelm IV of Prussia rejected the imperial crown that the Frankfurt Parliament offered, because it would have entailed his acceptance of a crown from a popularly elected assembly, which he deemed an unacceptable source of royal power. Ultimately, German unification was not achieved until much later under the leadership of Otto von Bismarck, culminating in the proclamation of the German Empire in 1871 after the Franco-Prussian War. It was only after this event that a constitution was established for the newly formed empire.