While the Zollverein itself was not a unifying force of the time, at least on its own, many things must be taken into account when questioning how far it futhered the road to unification. And to decide this, one must take look at its roots, and the reasons for its founding, before actually looking over its history. After the Napoleonic War, the various Germans states went into an industrial boom (especially after 1850) and some form of trade cohesion was going to be needed, if satsifactory trade were to take place within the states.One could hazard the comment that the need for such a thing was a precursor for unification, but it is a weak argument at best, with only a few supporters among the Historial Community. And although one cannot doubt that the original form of the Zollverein was very much the Kleindeutchland that Germany eventually came to be, it is still clear that the single states within the Zollverein were not neccessarily diplomatically linked, monetarily linked, or any such thing. This was seen clearly in 1866, when the Southern States rallied to aid Austria, despite their membership of the Zollverein.
It is also possible to say that, left alone, the Zollverein would have had no major effect. But like with so many areas of German government, Bismarck used it to achieve his ends, along with his predecessors who knew that a Zollverein without Autria’s involvement would give Prussia economic power in the Confederation, to match the political and military sway of Austria. After 1848 (after the many uprisings across the Austrian Empire, which left her cripplied economically) it became even more important to stop Austria from joining, as Prussia’s power within the Confederation continued to increase, and Bismarck did this the most actively, through a series of trade agreements with Belgium, England and Italy, which were based on the French traty of 1862. Bismarck used the Zollverein as one of the many tools in his foreign and domestic policy, to achieve his aims, yet remaining constantly under the spotlight as some National Liberal, who it became harder and harder to doubt or question, as time went on.
The battle for the Zollverein, and thus economic control of Germany, was also important when looking at the build-up to the Austro-Prussian War. Bismarck convinced France to agree that any negotiations on the extension of the Franco/Prussian Free-trade agreement, in reference to the smaller states, would have to go through Berlin. This, Bismarck thought, would increase their dependence on Prussia, and thus allow Bismarck to renegotiate terms when the Zollverein was renewed in 1865, getting rid of the smaller states’ veto powers and suchlike.
The other German states voted him out on this, making it impossible for him to use the Franco/Prussian treaty against them, and thus he switched tack. Still using the Zollverein, Bismarck threatened to dissolve it in 1865, and only renegotiate with the individual states, on acceptance of the Franco/Prussian treaty. He knew that there was no other choice, and watched in amusement as the Austrian attempt to form some kind of trading union failed, and on 12th October 1864 Bavaria, Wurrtemberg, Hesse-Darmstadt and Nassau all agreed toPrussia’s conditions for renewal of the Zollverein. And once again Austria was excluded, despite unrealistic promises to look into the matter in 1872, allowing Prussia to have a stronger power-base than it had ever had before. It is no chance that within two years, Austria and Prussia were to be at war, the growing economic power of Prussia clearly unsettling Austria’s claim to lead the German states.
Yet once again, it must be pointed out that the states of the Zollverein did not stick together miltarily. Many turned against Prussia, deciding to fight alongside Austria, and the Northern states that did (or just didn’t let Prussia in, such as Frankfurt) were annexed or heavily fined at the end of it all. And the Southern states, although allowed to keep their independence, were forced to join the Zollverein, thus increasing Prussia’s overall power within the Confederation – one could even go as far to say that it was the Zollverein that, indirectly, knocked Austria out of the running for leadership of Germany.
In conclusion, I don’t think one can over-estimate or under estimate the extent to which the Zollverein unified Germany. It certainly played a part in drawing up the future extents of the German Empire, and allowed for greater intergration within the separate states, but its years of most effectiveness ended in 1865, after the addition of the Southern states, who returned members of the Zollverein Parliament who were ready to rebuff any of Prussia’s plans. It was not only that, but also the fact that Bismarck found other tools to use to his advantage (such as the Ems telegram, to steer Prussia and Germany in the direction he wanted).