Answer:
Militarism: the Industrial revolution, which started in Britain in the late 18th century, had spread to several European countries by the early 20th century (Belgium, the Netherlands, Germany, France and Italy).
The increased industrial power meant that more weapons could be made, and these weapons were now more lethal too. Organization principles of factories were applied to the military as well, making armies more efficient and professional.
Alliances: European countries were forming alliances amongst themselves, which in turn were opposed to other alliances. For example, the Triple Alliance of Germany, Italy and Austria-Hungary, and the Tripe Entente of Russia, France, and Britain.
Imperialism: European powers had colonies, and in cases like France or Britian, these colonies were scattered all over the globe. These powers wanted to expand their colonial empires, frenquently at their expense of their European neighbors. This obviously led to conflict.
Nationalism: Nationalism was on the rise on European countries. German, French, British nationalism, and so on, represented a risk because it could easily lead to clash between neighbors.