Selfish Nationalism is blowing on the kindling. Pan Slavism was a type of nationalism in Russia, which supported the cause of the Slavic nations, as Russia was one of the most important Slavic nations in Europe. When Serbia asked Russia for help to fight Austria-Hungary, Russia had to comply, getting the major alliances involved in World War 1. .
Another strong example of the impact of nationalism on the First World War was the assassination of Archduke Francis Ferdinand. A Serbian nationalist murdered him. In revenge, Austria-Hungary declared war on Serbia. Russia ordered a partial mobilization against Austria-Hungary, which turned general. This was because Austria-Hungary was a part of the Triple Alliance. From this, you can see how nationalism indirectly led to this. In retaliation, Germany, another member of the Triple Alliance warned and then declared war on France and Russia. This was because both these countries were part of the Triple Entente and were therefore, viewed as enemies of the Triple Alliance. Nationalism first influenced the Serbian murderer and this chain of events followed the assassination. Thus, Nationalism plays a large role. You can see Sarajevo, where the Archduke was killed is drawn as a lighted match in Source A because it was the nationalist event nearest to the beginning of the First World War. (The burnt out matches - Bosnia Herzegovina, Algeciras, the Balkan Wars and Agadir - were all events that strained relationships between countries in the Triple Entente and Triple Alliance. In these events, countries displayed a selfishness and often provoked threats of war from other nations.) Whereas, the Triple Alliance and Triple Entente are both huge logs in the picture. That is because they played a huge role in the beginning of the First World War by the chain of events spurred by the single act of nationalism above. The unhealthy rise of nationalism in those days convinced the people to be ready to die for their countries in a war that was started without much motive.
