The unification of Germany (, ) was the process of building the modern German nation-state with federal features based on the concept of Lesser Germany (one without multinational Austria of the Habsburgs), which commenced on 18 August 1866 with adoption of the North German Confederation Treaty establishing the North German Confederation, initially a Prussian-dominated military alliance which was subsequently deepened through adoption of the North German Constitution. The process symbolically concluded with the ceremonial proclamation of the German Empire i.e. the German Reich having 25 member states and led by the Kingdom of Prussia of the Hohenzollerns on 18 January 1871; the event was later celebrated as the customary date of the German Empire's foundation, although the legally meaningful events relevant to the accomplishment of unification occurred on 1 January 1871 (accession of South German states and constitutional adoption of the name German Empire) and 4 May 1871 (entry into force of the permanent Constitution of the German Empire). Despite the legal, administrative, and political disruption caused by the dissolution of the Holy Roman Empire in 1806, the German-speaking people of the old Empire had a common linguistic, cultural, and legal tradition. European liberalism offered an intellectual basis for unification by challenging dynastic and absolutist models of social and political organization; its German manifestation emphasized the importance of tradition, education, and linguistic unity. Economically, the creation of the Prussian (customs union) in 1818, and its subsequent expansion to include other states of the German Confederation, reduced competition between and within states. Emerging modes of transportation facilitated business and recreational travel, leading to contact and sometimes conflict between and among German-speakers from throughout Central Europe. The model of diplomatic spheres of influence resulting from the Congress of Vienna in 1814–15 after the Napoleonic Wars endorsed Austrian dominance in Central Europe through Habsburg leadership of the German Confederation, designed to replace the Holy Roman Empire. The negotiators at Vienna took no account of Prussia's growing strength within and declined to create a second coalition of the German states under Prussia's influence, and so failed to foresee that Prussia would rise to challenge Austria for leadership of the German peoples. This German dualism presented two solutions to the problem of unification: , the small Germany solution (Germany without Austria), or , the greater Germany solution (Germany with Austria), ultimately settled in favor of the former solution in the Peace of Prague. Historians debate