why did nationalism had a variable effect on European and world politics

A strong resentment of what came to be regarded as foreign rule began to develop. In Ireland, Italy, Belgium, Greece, Poland, Hungary and Norway local hostility to alien dynastic authority started to take the form of nationalist agitation. Nationalism came where an eight-year war (1821–1829) against Ottoman rule led to an independent Greek state; Belgium obtained independence from the Netherlands. Over the next two decades nationalism developed a more powerful voice, spurred by nationalist writers championing the cause of nationalist self-determination. In 1848, revolutions broke out across Europe, sparked by a severe famine and economic crisis and mounting popular demand for political change. In Italy Giuseppe Mazzini used the opportunity to encourage a war mission: "A people destined to achieve great things for the welfare of humanity must one day or other be constituted a nation".

In Hungary, Lajos Kossuth led a national revolt against Austrian rule; in Transylvania, Avram Iancu led of the nationalist revolts in 1848 were successful, any more than the two attempts to win Polish independence from Russian rule in 1831 and 1846 had been. Conservative forces proved too strong, while the majority of the populations little understood the meaning of national struggle. But the 1848 crisis had given nationalism its first full public airing, and in the thirty years that followed no fewer than seven new national states were created in Europe. This was partly the result of the recognition by conservative forces that the old order could not continue in its existing form. Conservative reformers such as Cavour and Bismarck made common cause with liberal political modernizers to create a consensus for the creation of conservative nation-states in Italy and Germany. In the Habsburg empire a compromise was reached with Hungarian nationalists in 1867 granting them a virtually independent state. In the Balkans the Serbian example had inspired other national awakenings. Native history and culture were rediscovered and appropriated for the national struggle. Following a conflict between Russia and Turkey, the Great Powers met at Berlin in 1878 and granted independence to Romania, Serbia and Montenegro and a limited autonomy to Bulgaria.