How does nationalism account for the fact that World War I was both a result of imperialism and the beginning of the end of empires?

The imperial rivalries among Germany, Austria-Hungary, Russia, France, Great Britain, Italy, and the Ottoman Empire (Turkey) were the cause of World War I, but it was nationalism within a couple of these empires that pushed them into dangerous alliances and sparked the start of the war.
Both Austria-Hungary and Turkey had a variety of nationalities within their borders and needed to keep them under control to keep their empires intact. This was something rival empires could exploit; for example, the Russians made allies with Serbia as a means of protecting itself against Austria-Hungary; the Serbs were enemies of Austria, working to unite the Slavic people in the Balkan, where many were subjects of the Austrians. Austria-Hungary in turned sided with Germany to counter the Russian threat, as did Turkey. Turkey was contending with both Russian rivalry around the Black Sea and the Caucasus and British and French ambitions in Egypt and the Middle East, where there were Arab people looking to free themselves from the Turks. 
One act of aggression in these sensitive areas could cause the dominoes of the alliances to fall, and that is exactly what happened when the Austrian heir Franz Ferdinand was assassinated by a Serbia-backed Bosnian nationalist: Austria-Hungary moved against Serbia, which brought it into conflict with the Russians, which drew Austria's ally Germany into the fight. France was allied with Russia, Britain supported France, and the war between those two and the German-Austrian alliance drew in the Ottomans on the side of their ally Germany. Italy, seeing an opportunity to gain territory at the expense of Austria, its natural rival but ally of convenience, switched sides and attacked Austria. 
And it just so happened that the powers that were most threatened by nationalism within their borders before the war were on the losing side, so their empires collapsed quickly. The new freedom of people like the Slavs and the Arabs encouraged nationalists in the empires of Britain and France, starting a process that would dissolve their empires as well in time. 
An excellent source: Sir Basil Liddell Hart, A History of the First World War 
 
https://books.google.com.ph/books/about/A_History_of_the_First_World_War.html?id=aGDnAwAAQBAJ&redir_esc=y