Nationalism is an ideology and movement that promotes the interests of a particular nation (as in a group of people) especially with the aim of gaining and maintaining the nation's sovereignty (self-governance) over its homeland. Nationalism holds that each nation should govern itself, free from outside interference (self-determination), that a nation is a natural and ideal basis for a polity, and that the nation is the only rightful source of political power (popular sovereignty). It further aims to build and maintain a single national identity—based on shared social characteristics such as culture, language, religion, politics, and belief in a shared singular history[page needed]—and to promote national unity or solidarity. Nationalism, therefore, seeks to preserve and foster a nation's traditional culture, and cultural revivals have been associated with nationalist movements. It also encourages pride in national achievements, and is closely linked to patriotism.[page needed] Nationalism is often combined with other ideologies, such as conservatism (national conservatism) or socialism (socialist nationalism) for example. Throughout history, people have had an attachment to their kin group and traditions, to territorial authorities and to their homeland, but nationalism did not become a widely-recognized concept until the 18th century. There are three paradigms for understanding the origins and basis of nationalism. Primordialism (perennialism) proposes that there have always been nations and that nationalism is a natural phenomenon. Ethnosymbolism explains nationalism as a dynamic, evolutionary phenomenon and stresses the importance of symbols, myths and traditions in the development of nations and nationalism. Modernism proposes that nationalism is a recent social phenomenon that needs the socio-economic structures of modern society to exist. There are various definitions of a "nation", however, which leads to different strands of nationalism. Ethnic nationalism defines the nation in terms of shared ethnicity, heritage and culture, while civic nationalism defines the nation in terms of shared citizenship, values and institutions, and is linked to constitutional patriotism. The adoption of national identity in terms of historical development has often been a response by influential groups unsatisfied with traditional identities due to mismatch between their defined social order and the experience of that social order by its members, resulting in an anomie that nationalists seek to resolve. This anomie results in a society reinterpreting identity, retaining elements deemed acceptable and removing elements deemed unacceptable, to create a unified community. This development may be the result of internal structural issues or the result of resentment by an existing group or groups towards other communities, especially foreign powers that are (or are deemed to be) controlling them.National symbols and flags, national anthems, national languages, national myths and other symbols of national identity are highly important in nationalism. In practice, nationalism can be seen as positive or negative depending on context and individual outlook. Nationalism has been an important driver in independence movements, such as the Greek Revolution, the Irish Revolution, the Zionist movement that created modern Israel, and the dissolution of the Soviet Union. Conversely, radical nationalism combined with racial hatred was also a key factor in the Holocaust perpetrated by Nazi Germany. More recently, nationalism was an important driver of the controversial annexation of Crimea by Russia.