One cause of the outbreak of World War I was caused by
a complex web of alliances made between the major powers of Europe at the time. These alliances meant that when one country declared war, their allies would be obligated to join in the conflict.
The spark that ignited the war was the assassination of Archduke Franz Ferdinand of Austria-Hungary in June 1914 by a Serbian nationalist. Austria-Hungary, backed by Germany, saw this as an opportunity to assert their dominance in the region and issued an ultimatum to Serbia.
When Serbia did not comply with all the demands, Austria-Hungary declared war on Serbia on July 28, 1914. This triggered the alliance system, as Russia was bound by treaty to protect Serbia and began mobilizing its troops. Germany, allied with Austria-Hungary, declared war on Russia on August 1, 1914, and then on France on August 3, 1914, as it was bound by a treaty to support Austria-Hungary. The conflict had become a global war, with the involvement of other major powers like Britain, Italy, and the United States eventually joining on different sides.