Nationalism
Extreme pride in one's country-starting in the 19th century
Primary Source- Gavrilo Princip during his trial for the murder of Archduke Franz Ferdinand
"I am a Yugoslav nationalist, aiming for the unification of all Yugoslavs, and I do not care what form of state, but it must be freed from Austria."
Causes
Nationalism ultimately began after the Enlightenment, which encouraged national identity, but nationalism is usually sparked by either political/social unrest or the mistreatment and embarrassment of a nation as a whole. With the political/social unrest, when people are unhappy in their current situation there's always someone who tries to unite their nation under nationalism to repair the unrest. For example, when China was under attack by Japan the People's Republic of China was founded (1949) and lead the nationalism movement to pick up China's crushed spirits. With regards to the mistreatment or embarrassment of a nation, when a nation as a whole goes through a traumatic experience its leaders try to heal some of the pain by uniting the nation and making sure it never happens again. An example of this is China after The Century of Humiliation (1839-1949) when China began to stand up to Japan after being constantly tormented.
Effects
There are a lot of effects for nationalism but the most prominent are competition between nations, overthrowing of leaders, and it can also create war. When nations began to grow nationalistic, a sense of competition arose. Everyone wanted to be the hegemony, or at least one of the major world powers, which wasn't necessarily bad because it started a competition for scientific and technological advances as well-which led to a lot of helpful ideas and inventions(i.e. guns, tanks, and other wartime weapons). Nationalism can also lead to the overthrowing of leadership or governments. When nationalism takes over, generally all foreign power/influence is wanted out. For example, after World War 2 most of the African countries and a few Southeast Asian counties kicked out European colonial rule and became independent nations. When nationalism results in war, it's usual because of tensions caused by the desire for more land, a nation rebelling against foreign power, or the competitive attitude.
Examples
Post WW1 Germany
After the Treaty of Versailles was signed(1919), Germany was forced to reduce their army, return land to France, give up their oversea colonies, and accept blame for the war-which meant paying reparations. The treaty was hard on Germany, and the harshness was used as a tool to unify the German people. Later on, Hitler will tap into this anger the Germans felt towards the treaty and use it to nationalize them. With this new nationalistic Germany, Hitler was able to led them into World War 2 thinking they could easily win.
WW2 Russia
During WW2, Russia was tricked by Hitler which led to it joining with the Allies to defeat the Nazis. Russia had no help during the majority of the fighting. A vast amount of land was also destroyed from both a "scorched earth policy" and the numerous battles on Russian soil. Russia was forced to use the only resource they had-their large population. With the war efforts leaving many families broken and without a home, Russia needed to be unified and get back the drive to win the war and rebuild their country. Stalin united the Russian people under communism and kept the nationalistic spirits high allowing them to defeat the Nazis at Stalingrad, which would be the first German loss.
Austro-Hungarian Empire
The Austro-Hungarian Empire was made up of multiple ethnic groups(Hungarian, German, Czech, Slovaks, Croats, Poles, Serbs, Slavs, and Italians. A lot of the different groups wanted to be their own nation. The tension between the groups eventually led to the break-up of the Austro-Hungarian Empire and the creation of Austria, Hungary, Czech Republic, Slovenia, Slovakia, Montenegro, Bosnia and Herzegovina, Croatia, Ukraine, Romania, Poland, Serbia, and part of Italy.