The concept of solidarity can be traced back to the Latin adjective „solidus,“ which means „tight“ and „firm,“ to which the German adjective „solide“ still bears witness today. In Roman law, solidarity referred to the cohesion within a community and was associated with the idea of liability, according to which the members of a community were responsible for the member who could not pay his debts. It was not until the late 18th century that the concept was detached from its debt-law context.

In Germany, it received its classical expression through the workers‘ movement of the 19th century. The emerging labor force recognized relatively quickly that their common interest in higher wages and shorter working hours could be better realized as a group than individually. Accordingly, solidarity is practiced by those who commit themselves to the union’s undertakings instead of acting as wage depressors or strike breakers. It is no accident that the Polish trade union Solidarność bears the term in its name.

In the welfare state of the 20th century, the nation was increasingly understood as a community of solidarity. National solidarity in Germany manifests itself not only in redistributive measures between citizens (taxes and social security contributions), but also between states (state fiscal equalization and advance sales tax equalization) and entire regions (solidarity pact and solidarity surcharge). In fact, there is no other level at which the principle of solidarity is so institutionalized as at the nation-state level.

In the 21st century, solidarity also seems to have become one of the guiding principles of the European Union. The Treaty of Lisbon, adopted in 2007, refers to the concept of solidarity a total of 15 times. The Treaty on the Functioning of the European Union even contains a solidarity clause that obliges the Union and its member states to provide mutual assistance in the event of a terrorist attack or a natural or man-made disaster.