World War I was called the Great War because it involved over 100 countries and resulted in over 16 million deaths, including 10 million soldiers and 6 million civilians. The war destroyed three major empires and left over 50% of soldiers wounded or suffering from psychological trauma. It was caused by rising nationalism among ethnic groups seeking independence, imperialism and competition for colonies, growing militarism as countries built up their armed forces, and the rigid alliance system that pulled more countries into the war. Poor decision making by leaders also contributed to the conflict.