World War I was caused by rising nationalism in European nations and their competition for power and resources, which led to the formation of opposing alliances. Militarism and an arms race escalated tensions as countries expanded their armed forces. Immediate causes of the war included the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist and the chain reaction of countries honoring their alliance obligations by declaring war on each other. Within a month, the major European powers were at war across both Western and Eastern fronts.