Kaiser Wilhelm I played a crucial role in changing the power structure in Europe by unifying Germany, which transformed it into one of the most powerful nation-states on the continent. Through the efforts of Otto von Bismarck, his chancellor, Wilhelm I was instrumental in the series of wars that ultimately led to German unification, notably the Franco-Prussian War. This unification significantly shifted the balance of power in Europe, as a unified Germany emerged as a dominant force, overshadowing Austria and altering the dynamics of alliances and rivalries across Europe.
Therefore, the correct summary of how Kaiser Wilhelm I changed the power structure is:
He unified Germany to become one of the most powerful nation-states in Europe.