Simply put, nationalism is a strong connection to one's nation. Citizens swayed by nationalism focus on their own nation's interests over the interest of other nations. In the United States, for example, nationalism is interwoven throughout day-to-day life, from when students pledge allegiance to the flag to when the national anthem is sung in the beginning of sporting events.
Nationalism creates a sense of unity among members in a group. Throughout history, nationalism has been used to create strong ties between citizens of a country, especially during wartime. When Adolf Hitler rose to power in Germany, he used a form of extreme nationalism to inspire Germans to rally behind him and his Nazi beliefs. Hitler preached that Germans were the better, stronger, purer race among other races in the world. He instilled a sense of pride in the German nation after World War I, a period in which Germany was economically depressed.
In Denmark, the Danes are proud to be a part of their country. When celebrating birthdays, weddings, and other life events, Danes often decorate their homes with small Danish flags. When relatives visiting other countries return home, small danish flags are waved as they arrive at the airport. Nationalism in Denmark also appears during handball championships, when even citizens who do not follow the sport wave flags and wear jerseys to show their support for their team.