WWI was caused by militarism, imperialism, alliances and nationalism. Conflict began over land in the Ottoman empire when Bosnia wanted independence from Austria Hungary.
After the A.H heir to the throne was assassinated, countries throughout Europe were pulled into war because of their alliances together. The two alliances in WWI were called Allies and Central Powers. The Allies consisted of Russia, Great Britain, France, Serbia, Romania, Greece, Portugal and the United States, while the Central Powers consisted of Germany, Austria-Hungary, Italy, Bulgaria and the Ottoman Empire.
This map illustrates the alliance system in Europe during the start of WWI