Militarism.
Alliances.
Imperialism.
Nationalism.
These were the four main causes of World War One.
Militarism
Germany had the most powerful army, but it wanted a better navy. Britain thought this was a challenge, and there was competition between the Dreadnought battleships. (Britain had 29 and Germany had 17 in 1914.) Britain needed a bigger navy because it was an island, could be attacked by two countries at once, needed to bring food from oversea etc.
Alliances
There were 2: the Triple Entente (Great Britain, France and Russia), and the Triple Alliance (Germany, Italy, and Austria-Hungary).
Imperialism
Britain and France had huge empires. Germany was envious and wanted a ‘place in the sun’, to build up its wealth and importance as a country. Britain and France saw this as a threat. Additionally, Britain was losing its trading markets to Germany in 1900.
Nationalism
Russia and Austria-Hungary tried to extend their influence in the Balkans; in 1908, Bosnia-Herzegovina became a part of Austria-Hungary. Slav groups in Austria-Hungary also wanted independence, like Serbia, but hostility between Austria-Hungary and Serbia was at an all-time-high because of their defeat of Balkan states in the Balkan Wars in 1913.