Who started World War 1?
Gavrilo Princip in Sarejevo.
Princip assassinated Archduke Ferdinand of Austria who was of course leader of the Austrian-Hungarian Empire. Then Austria formed a military union with Germany before declaring war on Serbia. After that, Russia came to the defense of its ally, Serbia. And France had a long standing treaty with Russia which made it an instant enemy of Germany.
A bit over the top to blame one Nationalist assassin. Europe was well on its way to war before June 1914.
International tensions before the war include: Moroccan Crisis in 1905 and 1911 and the Bosnian Crisis in 1908 and the Balkan Wars in 1912 and 1913.
Europe was also an extensively armed with powerful new weapons technologies with huge conscript armies available. Nationalist sentiment ran high. Certainly there was an anticipation of a war.
Very few people anticipated the results of the War.
Barbara Tuchman's "Gun's of August" published 1962 (also published as "August 1914) a book the First World War explores reasons behind the beginnings of the war. There are many others worth reading but Ms. Tuchman was alive at the time of the war and its aftermath.
The causes of World War 1 were complex and that war led directly to the next World War. There are still lessons to be learned from the study of these times.
The street corner in Sarajevo has a plaque where the Assassination of the Archduke took place. Worth a visit if you go.