Final answer:
The unification of Germany and Italy disrupted the balance of power established by the Congress of Vienna, leading to increased tensions and the formation of alliances that contributed to the outbreak of World War I.
Step-by-step explanation:
The unification of Germany and Italy significantly affected the balance of power that had been established by the Congress of Vienna. After the fall of Napoleon, the Congress aimed to maintain a balance by restoring old boundaries and creating buffers around France. However, German unification in particular, under the leadership of Prussia, created a new power that upset this balance. With a large population, a robust economy, and military might, Germany shifted the dynamic within Europe. The same applies, to a lesser extent, to Italy. As these unified countries emerged, the power structure in Europe moved from cooperative diplomacy to competition, leading to tensions and alliances that paved the way for World War I.