Final answer:
To secure the support of southern Germans, Bismarck instigated the Franco-Prussian War. This conflict successfully united the German states under Prussian leadership, completing German unification.
Step-by-step explanation:
To gain the support of southern Germans for unification, Bismarck started the Franco-Prussian War. This conflict was instrumental in rallying the southern German states to the cause of unification under Prussian leadership. Fearing French imperialism and influenced by Bismarck's diplomatic maneuvers, including his successful isolation of France, the southern German states joined with the northern German Confederation. Bismarck's strategy was to use war to unify the German principalities, which had previously been divided in their loyalties between Prussia and Austria.
After the annexation of the northern German states, a conflict with Austria, the Austro-Prussian War, further solidified these states into a confederation with Prussia. But it was the Franco-Prussian War that garnered southern German support, enabling the establishment of a unified Germany under Prussian leadership in 1871.