What or who was responsible for World War I?
The trigger for World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. He was the heir to the throne of Austria-Hungary. That alone doesn't really explain why the war happened, though. Europe was ready for trouble.
The assassin was a Serb who was a citizen of Austro-Hungary but was very pro-Serbia. Austria-Hungary basically took this event as its chance to impose its will on Serbia. It said the Serbian government was involved and sent really arrogant demands to Serbia. Basically, they expected Serbia to say 'No way' and then they would have an excuse send their military.
At that point, one thing led to another because of the alliances (promises of support) that different countries had made to each other.
Austria-Hungary declared war on Serbia, and so Russia had to help Serbia since they had a treaty. Since Russia was involved, France had to help Serbia too. Since Austria-Hungary was involved, Germany had to help them. Germany came through Belgium to get to France, which meant Britain had to get involved. It kept going from there until much of the world was involved.