The term “irredentism” is used to describe the belief that a territory within another country’s borders should actually be part of a nation that exists either outside of or independent of those borders. Generally, in European history, it refers to a territory that was once part of a nation, such as France with Alsace-Lorraine or Germany with Austria, that has been lost in conflict and which the losing nation desires to reclaim. It is one of the more dangerous and prominent forms of European nationalism because it so often leads to war in European history.

At the height of its reach, the Austro-Hungarian Empire occupied much of Central Europe and extended south into the Balkans and East into Eastern Europe. It was composed of several distinct ethnic groups, all of whom had a shared history, language, and cultural identity. This had long been the composition of the empire, but with rising nationalism in the nineteenth century, this became a problem that was more difficult to manage. All of these ethnic groups expressed nationalist ambitions in the nineteenth century except for the Poles, who lived outside of the Austro-Hungarian Empire and were at the time primarily controlled by Russia and Germany. Each of these ethnic groups currently has its own nation-state, with all of them being self-explanatory except perhaps the Magyars (Hungary).

The Act of Union of 1801 dissolved the Irish Parliament and formally ended any semblance of Irish independence. The territory of Ireland was incorporated into the United Kingdom, now called the United Kingdom of Great Britain and Ireland. The Act of Union may be understood as a British government reaction to the Irish Rebellion of 1798 and also a response to the French Revolution a decade earlier, both of which inflamed Irish nationalism.

The Kingdom of Piedmont-Sardinia was one of the most influential and economically prosperous by the time the movement towards Italian unification began in the mid-nineteenth century. Led by Victor Emmanuel and Camillo di Cavour, the Kingdom was the primary driving force behind Italian unification. In 1861, the Kingdom of Sardinia annexed all the other territories that comprise modern-day Italy into the Kingdom of Italy.

"Irredentism" comes from the Italian phrase "Italia irredenta," which means "unredeemed Italy" and referred to those culturally and linguistically Italian regions ruled by Austria that Italian nationalists wanted to be part of a Unified Italy. Irredentism was also a key component in German unification, French nationalism, and pan-Slavic movements. Remarkably for the period, irredentism also had few ties to particular forms of government, since it was tied to ethnic, cultural, and linguistic affinities.

Garibaldi was one of the most famous revolutionaries of the mid-nineteenth century. His revolutionary career began in South America as an agitator and soldier in favor of various republican causes. He returned to Europe in the midst of the revolutions of 1848. He raised an army of volunteers to fight the Austrian Empire in order to free Northern Italy so that the entire Italian peninsula could be unified. Garibaldi was one of the central figures of the Italian Risorgimento, which resulted in the creation of the Kingdom of Italy in 1861.

Giuseppe Garibaldi was an Italian military commander who lived from 1807 until 1882 and who fought in a variety of conflicts meant to bring about the unification of Italy. He was a trusted general and advisor of Victor Emmanuel II, the first king of Italy, and is credited with helping him achieve a mostly unified Italy. Garibaldi was widely respected for his renowned military mind and was fairly popular. During the fight to unify Italy, he had to rely mainly on volunteers who became known as “Redshirts” because they lacked the money to buy uniforms and instead they wore red. These volunteers helped not only by fighting, but also by showing that this movement had popular support among the people.

During the rise of nationalism in Europe—a process begun around the sixteenth century and culminating in the World Wars of the twentieth century—the most important factor for determining shared national identity was a shared language. This is how German nationality arose from the scatterings of Germanic people around Europe—they often spoke the same root language. The same is true in Italy, France, England, and so on.

Camillo di Cavour was chosen as the Prime Minister of the Kingdom of Piedmont-Sardinia (in Northern Italy) by the King Victor Emmanuel II in 1852. Cavour was a dedicated statesman who used his position to push for economic expansion and, subsequently, the political expansion of his kingdom. By 1871, Italian unification had been achieved.

At the beginning of the nineteenth century, each of these nations was either a part of a larger empire or split into many different kingdoms and republics except for Russia, which already had a centralized government that reflected the people of the same nation.