Final answer:
The balance of power in Europe shifted from a post-Napoleonic era of peace established by the Congress of Vienna to a dynamic power play after the unification of Germany, which marked the beginning of competition among European nations.
Step-by-step explanation:
From 1815 to 1870, the balance of power in Europe underwent significant changes, beginning with efforts to restore stability after the Napoleonic wars. The Congress of Vienna in 1815 was instrumental in redrawing borders and establishing a balance that favored peace and the status quo, with Prussia, Austria, and Russia all gaining territory. However, by 1870, the rise of nationalism and the unification of Germany disrupted the existing order. Germany's emergence as a powerful nation with a strong industrial base and military upended the carefully constructed balance put in place by the Congress of Vienna, leading to a period of competition among the European powers.