The accession of the city state of Hamburg to the German Customs Union, commonly known as Zollverein, in 1888 (along with Bremen) was the culmination of a project for the economic and monetary union of Germany, stretching back to 1819. In that year Schwarzburg-Sondershausen joined Prussia’s internal customs union, the first other state to do so and the first of many to follow. [1]
When the German Customs Union was initially formed, in the 1830s and 1840s, the motives of the states in joining did not include any particular desire to unite with others to form a wider more powerful entity. The states which joined fought vigorously against any impairment of their full sovereignty. Hence each member had a veto and the union was not permanent, but had to be renewed after 12 years. Each member sought its own economic advantage. Accordingly, the accession of any of the Hanseatic cities of Hamburg, Bremen or Lübeck would have to be on condition that accession served the city's own advantage. A pre-condition of such advantage was that their territory touched directly with that of the Customs Union, which only happened for Hamburg after 1854, when Hanover joined. However Hamburg kept an open frontier with Holstein until 1867, when it was included in the Customs Union on being annexed to Prussia. But, after these accessions, other factors came into play as regards the question of Hamburg joining.[2]