Causes of World War 1 World War I was the result of leaders' aggression towards other countries which was supported by the rising nationalism of the European nations. Economic and imperial competition and fear of war prompted military alliances and an arms race, which further escalated the tension contributing to the outbreak of war. For Twenty years, the nations of Europe had been making alliances. It was thought the alliances would promote peace. Each country would be protected by others in case of war. making it foolish for one country to w ....