That would be nationalism in the Balkans. Prior to WWI, Austria-Hungary had annexed Bosnia and Herzegovina, angering many in the Balkans especially those who wished to see a unified Yugoslavia. One such group was the Young Bosnia revolutionary organization, whose membership included Gavrilo Princip, the man who assassinated Archduke Franz Ferdinand in 1914, thus starting WWI.
World War I had many causes. Among them was the growing nationalism in many areas of the world. Peoples of the Balkan Peninsula, for example, experienced a surge of nationalism in the years before the war. Many areas of the Balkans were under the control of large empires, including the Ottoman and the Austro-Hungarian empires. The people of the Balkans believed that they should be able to govern themselves and began to fight against the rule of the controlling empires.
The primary reason samurai adopted Zen Buddhism was the belief that it strengthened them on the battlefield. They believed that plans and strategy on the battlefield led to death, and Zen helped followers dismiss such thoughts.
