Even in the earlier stages of its development the Zollverein had a marked effect on the condition of the country. Its growth coincided with the introduction of railways, and enabled the nation to derive from them the full benefit; so that, in spite of the confusion of political powers, material prosperity increased, together with the consciousness of national unity and a tendency to look to Berlin rather than to Vienna as the centre of this unity.
This tendency was increased by the accession to the throne of Prussia, in 1840, of Frederick William IV., a prince whose conspicuous talents and supposed "advanced" views raised the hopes of the German Liberals in the same degree as they excited the alarm and contempt of Metternich. In the end, however, the fears were more justified than the hopes. The reign began well, it is true, notably in the reversal of the narrow ecclesiastical policy of Frederick William III. But the new king was a child of the romantic movement, with no real understanding of, and still less sympathy with, the modern Liberal point of view. He cherished the idea of German unity, but could conceive of it only in the form of the restored Holy Empire under the house of Habsburg; and so little did he understand the growing nationalist temper of his people that he seriously negotiated for a union of the Lutheran and Anglican churches, of which the sole premature offspring was the Protestant bishopric of Jerusalem.
Meanwhile the Unionist and Liberal agitation was growing in strength, partly owing to the very efforts made to restrain it. The emperor Nicholas I. of Russia, kept informed by his agents of the tendencies of opinion, thought it right to warn his kinsman of Prussia of the approach of danger. But Frederick William, though the tsar's influence over him was as great as over his father, refused to be convinced. He even thought the time opportune for finishing "the building begun by Papa" by summoning the central assembly of the diets, and wrote to the tsar to this effect (December 31, 1845); and he persevered in this intention in spite of the tsar's paternal remonstrances. On the 13th of February 1847 was issued a patent summoning the united diet of Prussia. But, as Metternich had prophesied, this only provided an organ for giving voice to larger constitutional aspirations. The result was a constitutional dead-lock; for the diet refused to sanction loans until its "representative" character was recognized; and the king refused to allow "to come between Almighty God in heaven and this land a blotted parchment, to rule us with paragraphs, and to replace the ancient, sacred bond of loyalty." On the 26th of June the diet was dissolved, nothing having been done but to reveal the widening gulf between the principle of monarchy and the growing forces of German Liberalism.