Austria, though a leading German state in 1815, disliked the idea of a united Germany since it would put her in a very embarrassing position. Although the Habsburg was of German origin, Austria was reluctant to become part of a united Germany because it would oblige her to give up her vast non-German population and territories in the Austrian Empire. What is more, she even opposed the idea of a united Germany since it would create a strong neighbor next to her. This was by no means desirable for Austria politically or strategically. Besides, the success of German Unification might lead to the disintegration of the Austrian Empire since it would stimulate the independence movements among those national minorities living under Austrian rule. Therefore, the multi-national nature of the Austrian Empire had dictated the policy of Austria towards the movements of German Unification.
Apart from this, the weaknesses of Austria and the conservatism of her statesmen also explained why she Congress of Vienna, Austria had been very much weakened by financial and racial problems within her empire. On the other hand, the conservative Austrian leaders also detested the ideas of nationalism and changes. All they would like to do was to maintain the status quo. As a result, the unification of Germany, which might cause so much trouble and danger to the Austrian Empire, had to be suppressed bluntly.
As a result of these considerations, Austria had made a series of attempts to suppress German Unification after 1815. Student movements were suppressed in late 1810s and the Carlsbad Decrees were passed to check the growth of nationalism in Germany. Besides, she also took the lead in suppressing the German revolutions in 1830 and 1848. Metternich also attempted to check the growth of nationalism in Germany and Europe by making use of the Congress System. In fact, Austria would even risk a war to prevent the success of German Unification. In 1849, when the Frankfurt Parliament offered the crown of united Germany to Prussia, Austria protested bitterly and threatened war on Prussia. War was only avoided when Prussia declined the throne. But a war could no longer be avoided when Prussia was determined to unify Germany in 1866. The Austro-Prussian War was the last attempt made by Austria to prevent the materialization of German Unification. Despite all these attempts, Germany was finally unified in 1871 under the leadership of Prussia.
Although Austria was determined to suppress German Unification, her opposition was finally overpowered by the mighty Prussians. With her internal decline especially after 1848, Austria could no longer stop the ambitious and powerful Prussia from unifying Germany. In fact, the success of Prussia was very much enhanced by her ever-growing strength since 1815. As a result of the Vienna Settlement, the size of Prussia was almost doubled and she became one of the two leading German states. On the other hand, Prussia benefited immensely from the Zollverein. With the abolition of customs dues, industries and commerce among the members grew rapidly. Prussia, being the leader of the Zollverein, was able to reap a lion share of these economic benefits. Apart from economic benefits, which greatly improved her military strength, Prussia also gained much administrative experience and prestige. As a consequence, the leading position of Prussia among the German states had been firmly established by the 1850s. The Zollverein had in fact laid the foundation for the later political unification of Germany in 1871.
Besides economic strength, the military strength of Prussia, which was so essential in clearing the way for unification, must not be overlooked. Under the leadership of William I and Bismarck, a vast program of military reforms was carried out to strengthen the Prussian army. The army was expanded and intensively trained, better equipment was installed and strategic railways built. With a powerful army at hand, nothing could stop Prussia from unifying Germany After a series of military victories in 1864, 1866 and 1871, Prussia finally accomplished the task of unification in 1871.
Of course, the diplomatic genius of Bismarck should also be credited. Through skilful manipulations, Bismarck succeeded in isolating the enemies before every war and this certainly helped to bring victories to Prussia. The joint action with Austria against Denmark, the handling of the Schleswig-Holstein issue and the Ems Telegram showed nothing but the diplomatic genius and skilful manipulations of Bismarck in capitalizing every possible chance to fulfill his goals. As a result, Prussia was able to accomplish the task of unification under the brilliant leadership of Bismarck.
Apart from the strength of Prussia, the rise of German nationalism in this period also contributed much to the success of German Unification. German nationalism had been greatly stimulated by the Napoleonic Wars. Despite the suppression of the Powers during the Congress of Vienna and the Congress System, the demand for national unity increased steadily among the Germans largely due to the work of revolutionaries and intellectuals. The development of the Zollverein and the legacy of the 1848 Revolution in Germany helped to push nationalism to a new height. The popularity of German nationalism in the 1860s certainly smoothed the way for Prussia in unifying the country.
Last but not least, the Unification of Germany in 1871 was also made possible by favorable international situations. With the decline of Austria, Prussia was able to remove the major obstacle to the Unification of Germany (i.e. opposition of Austria) with relative ease. Besides, the ambitions of other powers e.g. Denmark and France were also manipulated by Bismarck to clear the way for unification. By mobilizing the threat of foreign invasion, Bismarck succeeded in inducing the German states to join the cause of Prussia. The support of Italy and Russia as well as the neutrality of Britain in the course of unification also contributed to its final success in 1871.
All in all, despite the bitter opposition of Austria, Germany was finally unified under the leadership of Prussia. The decline of Austria, the strength and capable leadership of Prussia, the rise of nationalism and favorable international situations were the major reasons accounting for the success of German Unification in 1871.