The Unification of Germany (German: Deutsche Einigung, pronounced [ˈdɔʏtʃə ˈʔaɪnɪɡʊŋ] ^ⓘ) was a process of building the first nation-state for Germans with federal features based on the concept of Lesser Germany (one without the Habsburgs' multi-ethnic Austria or its German-speaking part). After the deletion of the German Confederation due to the Austro-Prussian War, it commenced on 18 August 1866 with the adoption of the North German Confederation Treaty establishing the North German Confederation, initially a military alliance de facto dominated by the Kingdom of Prussia which was subsequently deepened through adoption of the North German Constitution.

The process symbolically concluded when most of the south German states joined the North German Confederation with the ceremonial proclamation of the German Empire (German Reich) having 25 member states and led by the Kingdom of Prussia of Hohenzollerns on 18 January 1871; the event was typically celebrated as the date of the German Empire's foundation, although the legally meaningful events relevant to the completion of unification occurred on 1 January 1871 (accession of South German states and constitutional adoption of the name "German Empire"), 4 May 1871 (entry into force of the permanent Constitution of the German Empire) and 10 May 1871 (Treaty of Frankfurt and recognition of the Empire by the French Third Republic).

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 Zollverein (customs union) in 1818, and its subsequent expansion to include other states of the Austrian (under Austrian Empire)-led 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 1814–1815 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 underestimated Prussia's growing internal strength and declined to create a second coalition of the German states under Prussia's influence, and so failed to foresee that Prussia (Kingdom of Prussia) would rise to challenge Austria for leadership of the German peoples. This German dualism presented two solutions to the problem of unification: Kleindeutsche Lösung, the small Germany solution (Germany without Austria), or Großdeutsche Lösung, the greater Germany solution (Germany with Austria or its German-speaking part), ultimately settled in favor of the former solution in the Peace of Prague.

Historians debate whether Otto von Bismarck—Minister President of Prussia—had a master plan to expand the North German Confederation of 1866 to include the remaining independent German states into a single entity or simply that he planned to expand the power of the Kingdom of Prussia. They conclude that factors other than the strength of Bismarck's Realpolitik led a collection of early modern polities to reorganize their political, economic, military, and diplomatic relationships in the 19th century. Reaction to Danish and French nationalism prompted expressions of German unity. Military successes—especially those of Prussia—in three regional wars generated enthusiasm and pride that politicians could harness to promote unification. This experience echoed the memory of mutual accomplishment in the Napoleonic Wars, particularly in the War of Liberation of 1813–1814. By establishing a Germany without multi-ethnic Austria (under Austria-Hungary) or its German-speaking part, the political and administrative unification of 1871 avoided, at least temporarily, the problem of dualism.

Despite undergoing in later years several further changes of its name and borders, overhauls of its constitutional system, periods of limited sovereignty and interrupted unity of its territory or government, and despite dissolution of its dominant founding federated state, the polity resulting from the unification process continues today, surviving as the Federal Republic of Germany.

Germans emerged in medieval times among the descendants of the Romanized Germanic peoples in the area of modern western Germany, between the Rhine and Elbe rivers, particularly the Franks, Frisians, Saxons, Thuringii, Alemanni, and Baiuvarii. The region was divided into long-lasting divisions, or "Stem duchies", based upon these ethnic designations, under the dominance of the western Franks starting with Clovis I, who established control of the Romanized and Frankish population of Gaul in the 5th century, and began a new process of conquering the peoples east of the Rhine. In subsequent centuries the power of the Franks grew considerably. By the early 9th century AD, large parts of Europe had been united under the rule of the Frankish leader Charlemagne, who expanded the Frankish Empire (Francia) in several directions including east of the Rhine, where he conquered Saxons and Frisians. A confederated realm of German princedoms, along with some adjacent lands, had been in existence for over a thousand years; dating to the Treaty of Verdun i.e. the establishment of East Francia from eastern Frankish Empire in east of the Rhine in 843, especially when the Ottonian dynasty took power to rule East Francia in 919. The realm later in 962 made up the core of the Holy Roman Empire, which at times included more than 1,000 entities and was called the "Holy Roman Empire of the German Nation" from 1512 with the Diet of Cologne (new title was adopted partly because the Empire lost most of its territories in Italy and Burgundy to the south and west by the late 15th century, but also to emphasize the new importance of the German Imperial Estates in ruling the Empire due to the Imperial Reform). The states of the Holy Roman Empire ranged in size from the small and complex territories of the princely Hohenlohe family branches to sizable, well-defined territories such as the Electorate of Bavaria, the Margraviate of Brandenburg or the Kingdom of Bohemia. Their governance varied: they included free imperial cities, also of different sizes, such as the powerful Augsburg and the minuscule Weil der Stadt; ecclesiastical territories, also of varying sizes and influence, such as the wealthy Abbey of Reichenau and the powerful Archbishopric of Cologne; and dynastic states such as Württemberg. Among the German-speaking states, the Holy Roman Empire's administrative and legal mechanisms provided a venue to resolve disputes between peasants and landlords, between jurisdictions, and within jurisdictions. Through the organization of imperial circles (Reichskreise), groups of states consolidated resources and promoted regional and organizational interests, including economic cooperation and military protection.^{[citation needed]}

Since the 15th century, with few exceptions, the Empire's Prince-electors had chosen successive heads of the House of Habsburg from the Duchy of Austria to hold the title of Holy Roman Emperor. Although they initially sought to restore central Imperial power, preserving a weak and fragmented Empire was convenient for France and Sweden, and therefore, their ensuing intervention led to the Peace of Westphalia which effectively precluded any serious attempts to reinforce the imperial central authority and petrified fragmentation and leading to the existence of more than 300 German-speaking political entities, most of them being parts of the Holy Roman Empire, as the Napoleonic Wars dawned. Still though, portions of the extensive Habsburg Monarchy (exclusively its large non-German-speaking territories: Lands of the Crown of Saint Stephen and the Austrian partition of Polish-Lithuanian Commonwealth) or of the Hohenzollern Kingdom of Prussia (both the German-speaking former Duchy of Prussia and the non-German-speaking entire territory of the Prussian partition of Polish-Lithuanian Commonwealth) as well as the German-speaking Swiss cantons were outside of the Imperial borders. This became known as the practice of Kleinstaaterei ("small-statery"). As a further consequence, there was no typical German national identity as late as 1800, mainly due to the highly autonomous or semi-independent nature of the princely states; most inhabitants of the Holy Roman Empire, outside of those ruled by the emperor directly, identified themselves mainly with their prince rather than with the Empire or the nation as a whole. However, by the 19th century, transportation and communication improvements started to bring these regions closer together.

Invasion of the Holy Roman Empire by the First French Empire in the War of the Second Coalition (1798–1802) resulted in a massive military defeat for the Empire's and allied forces at the hands of Napoleon Bonaparte. The treaties of Lunéville (1801) and the Mediatization of 1803 secularized the ecclesiastical principalities and abolished most free imperial cities and so these territories along with their inhabitants were absorbed by dynastic states. This transfer particularly expanded the territories of Württemberg and Baden. In 1806, after a successful invasion of Prussia and the defeat of Prussia at the joint battles of Jena-Auerstedt 1806 during the War of the Third Coalition, Napoleon dictated the Treaty of Pressburg which included the formal dissolution of the Holy Roman Empire and the abdication of Emperor Francis II from the nominal reign over it. Napoleon established instead a German client state of France known as the Confederation of the Rhine which, inter alia, provided for the mediatization of over a hundred petty princes and counts and the absorption of their territories, as well as those of hundreds of imperial knights, by the Confederation's member-states. Several states were promoted to kingdoms including the Kingdom of Bavaria, the Kingdom of Saxony or the Kingdom of Hanover. Following the formal secession from the Empire of the majority of its constituent states, the Emperor dissolved the Holy Roman Empire. In his abdication, Francis released all former estates from their duties and obligations to him, and took upon himself solely the title of Emperor of Austria, which had been established since 1804.