Militarism.
As the world entered the 20th century, an arms race had begun. By 1914, Germany had the greatest increase in military buildup. Great Britain and Germany both greatly increased their army size in this time period as well. Further, in Germany and Russia particularly, the military establishment began to have a greater influence on public policy.  Thus the increase in militarism helped push the countries involved into WWI. What is militarism?​