Trouble Brewing
Militarism
Many Europeans were strengthening their militaries to be able to hold their own, should any other country attempt to attack. Other European states were seeing this preparation as a threat and began to build their militaries as well. All of this military improvement caused tension in Europe, and were a prime cause of WWI. When Germany challenged Britain's dominance of the seas, the tension grew to be too much and war broke out.
Alliances
Alliances were important in WWI because if you had an ally that another country declared war on, you are automatically part of that war on the side of your ally. Alliances are what made the Great War a world war.
Imperialism
Imperialism in Africa was another factor in the start of World War I. France and Britain owned many colonies in Africa, which increased the rivalry with Germany, who had a later start on the colonization of Africa, thus having less colonies.
Nationalism
After Napoleon was exiled, The Congress of Vienna came together to sort out problems in Europe. Ambassadors from Austria, Britain, Prussia and Russia agreed on new boundaries that divided Germany and Italy, but nationalism of citizens in those countries led to the reuniting of Italy in 1861 and Germany in 1871. Austria-Hungary and Serbia both housed different nationalist groups, all of which wished for freedom from the countries where they lived.