Main Causes of WWI Explained in 90 Seconds 

The 4 main causes of World War I were Militarism, Alliances, Imperialism, and Nationalism. WWI broke out after the assassination of Archduke Franz Ferdinand triggered the July Crisis, but these causes created the environment that allowed that single event to spark an all-out world war.
Sources:
Gardner, Robert, Daniel J. McDevitt, Margaret Hoogeveen, and Angus L. Scully. Exploring Nationalism. Toronto, ON: McGraw-Hill Ryerson, 2008.
Martel, Gordon. 2008. Origins of the First World War: Revised 3rd Edition. 3rd ed. London, England: Routledge.
Image Sources:
Alliance Map, By historicair (French original)Fluteflute & User:Bibi Saint-Pol (English translation) - Translated in English from French SVG Map_Europe_alliances_1914-fr.svg, CC BY-SA 2.5, commons.wikime...
Schlieffen Plan, By Lvcvlvs - Own work, CC BY-SA 3.0, commons.wikime...