The answer is nationalists and revolutionaries.
Nationalists and revolutionaries were indeed a destabilizing force in many parts of Europe after the peace was reestablished by the Congress of Vienna. The Congress aimed to restore the old order and suppress revolutionary movements, but the desire for national self-determination and liberal reforms continued to grow, leading to various uprisings and revolutions throughout the 19th century.
While the memories of Napoleon's conquests and reforms did influence European politics, they were not as directly destabilizing as the active movements of nationalists and revolutionaries.
The Congress of Vienna did redraw the map of Europe, but the borders were relatively clear. The main issue was that these borders often did not align with the nationalistic aspirations of various ethnic groups, which fueled nationalist and revolutionary movements.
Although some of the reinstituted ruling classes were politically weak, this weakness was more a consequence of the pressures from nationalists and revolutionaries rather than a primary destabilizing force.
The victor nations, such as Austria, Prussia, and Russia, were more focused on maintaining the status quo and suppressing revolutionary movements rather than being a destabilizing force themselves.
The answer is as the final stage in German unification.
Bismarck used wars with neighboring countries strategically to achieve German unification. The wars against Denmark, Austria, and France were instrumental in consolidating the various German states under Prussian leadership, culminating in the proclamation of the German Empire in 1871.
While Bismarck did aim to annex Schleswig and Holstein, this was part of his broader strategy for German unification rather than the sole reason for the wars.
Bismarck did not aim to destroy the North German Confederation; rather, he sought to strengthen and expand it into a unified German Empire.
Bismarck already held significant power in Prussia as the Prime Minister. His primary goal was the unification of Germany under Prussian dominance, not merely consolidating his power within Prussia.