Austria Hungary's Archduke Franz Ferdinand was assassinated by a Serbian group the Black Hand.
There He Is!
Austria Hungary declared war on Serbia claiming that Serbia assinated their archduke. Austria Hungary and Serbia meaning to have a small, short war, but causing a chain reaction of countries joining.
Germany joined the war as an ally with Austria Hungary, also wanting more power and international influence. Germany started a naval arms race with Britain who in response, built many warships like the Dreadnaught.
Russia, Joined the war with Serbia because of a treaty with Serbia, and because they needed to restore national respect because of their civil strifes and a loss to Japan in 1905.
France joined the war because of a treaty with Russia. Germany declared war on France on August 3, 1914. Germany then attacked neutral Belgium to reach Paris by a shorter route. France by extenstion was also at war with Austria Hungary. Britain, allied to France, was "obligated" to defend France. Britain's real reason was to defend neutral Belgium. Britain declared war on Germany on August 4, 1914. Britain was also by extension at war with Austria Hungary. Japan, keeping a military agreement with Britain, declared war on Germany on August 23, 1914. Austria Hungary responded by declaring war on Japan 2 days later. Italy, allied to Germany and Austria Hungary, stayed out of the war because they said that their actions were offensive and they would only join them in defensive. A year later Italy would join the war on May, 1915, siding with the allies. In 1917 the United States would join the war because of they German's use of submarines of bombing U.S. supply ships.
Allies: Serbia Russia France Britain Japan Italy United States