In the 1800's nationalism, imperialism, and militarism had a big part in the cause that started World War 1. Making territory, control, and trust an issue.
In 1871 Kaiser Wilhelm 1 sought to unite German-speaking people in Europe under Germany through the pan-German movement. Nationalism helped in enlarging Germany yet threatened the stability of other nations.
This led the Serbs wanting to create a nation of it's own. They encouraged the Slavs to join them to form an independent Slavic empire in the Balkan region. Russia then supported Serbia, making the Balkan region a likely place to ignite European war. .
As people competed to gain territories and build overseas empires, tension within the European countries grew. In the late 1800's, Great Britain had formed the largest imperial power. France, Germany, Italy, and Russia wanted to create similar empires as the one Great Britain had formed.
Nations then turned to militarism, which was a policy of aggressive military preparedness. In 1912 and 1913, Germany formed the most powerful military by increasing its army by more than 20 percent.
Balance of power was introduced to the European nations in the late 1800's to create alliances. In 1882, a triple alliance was formed between Austria-Hungary, Germany, and Italy to support each other in the case of an attack. Fearing Germany's growing power, Britain, France, and Russia formed a triple entente in 1907.
Triple alliance and entente then created a complicated balance of power in Russia.
On July 28, 1914, Austria-Hungary declared war on Serbia after a member of the Black Hand violence group, murdered Austria-Hungary's heir to the throne of and his wife. In support of Austria-Hungary, Germany declared war on Russian and then France. Britain then declared war on Germany to defend Belgian neutrality. On Aug. 5th, Austria-Hungary declared war on Russia, beginning to what we know now as World War 1.