World War I began in 1914. There were several factors that led to the start of the war. The factor that led directly to the start of the war was when a group of Serbian nationals assassinated Franz Ferdinand who was supposed to be the next King of Austria-Hungary.
After the assassination, Austria-Hungary made demands on the Serbian government. When the Serbian government couldn’t meet the demands, Austria-Hungary asked Germany if Germany would support them if they declared war on Serbia. Germany agreed, and Austria-Hungary declared war on Serbia. Russia, which was very supportive of Serbia, declared war on Austria-Hungary. Germany declared war on Russia. France, who had an alliance with Russia, declared war on Germany. World War I had begun. Eventually, other nations joined the war including Great Britain, Italy, and the United States.