The main causes of World War One can be broken down into four words:
Militarism is an intense pride and power of a countires military. All of the opposing countries were building up their armies, attempting to be the biggest and the most powerful. When these powers clashed, it resulted in the battles that shaped WWI.
Many of the countries had alliances with other countries, and because of their loyalties and respect of the country, were sometimes dragged into battle, rather than wanting to be in the battle. Example: If country A has an alliance with country B, then country B is expected to wage war with any country that country A is waging war with, even if country B has no problem with the opposing country.
Imperialism was the domination of a country by another country. All of the countries involved in WWI were expanding, and by imperialism, wanted to take over other countries for colonies, room, rescources, and expansion. With all of this tension already existing, with war as an excuse, some countries looked to dominate and expand, as well as win the war.
Nationalism is an extreme feeling of pride for one's country. With the militaries country men and women supporting them, and the country, the countries were extremely competetive with one another. Along with imperialism, and all of the other causes together, nationalism was another chief cause of WWI.