Colonialism: The subjugation and exploitation of colonized peoples created resentment and bitterness among colonized people. This fueled anti-colonial movements and contributed to the growth of nationalism in the colonies.
Nationalism: The rise of nationalism in Europe led to increased competition for power and prestige among European nations. This competition was exacerbated by the arms race, which saw the major European powers building up their militaries in preparation for war.
Militarism: The arms race and the growth of militarism in Europe created an atmosphere of fear and suspicion among the European powers. This made it more difficult to resolve conflicts peacefully.
Alliance system: The system of alliances between European nations made it difficult to localize conflicts. If one country went to war, its allies were obligated to come to its aid, which could quickly lead to a wider war.
Balkan nationalism: The Balkans was a region of southeastern Europe that was home to many different ethnic groups. As nationalism grew in the Balkans, ethnic tensions increased and the region became increasingly unstable.
The assassination of Archduke Franz Ferdinand: The assassination of Archduke Franz Ferdinand of Austria by a Serbian nationalist in June 1914 was the immediate trigger that set off World War I.