Imperialism:
European countries had been competing for colonies and resources around the world since the 18th century. By the early 20th century, the major powers had largely divided up the world between them. This competition for colonies led to increased tension and rivalry between countries.
Nationalism:
Nationalism, or the belief in the superiority of one's own country, became a powerful force in Europe in the 19th and early 20th centuries. This led to increased competition and conflict between countries, as each sought to assert its dominance and protect its national interests.
Militarism:
The major European powers engaged in an arms race in the years leading up to World War 2. They built up their militaries and developed new weapons, such as tanks, airplanes, and submarines. This arms race created a sense of insecurity and suspicion among countries, as each feared being attacked by its rivals.
Failure of the League of Nations:
The League of Nations was an international organization that had been created after World War 1 to prevent future conflicts. However, the League was unable to prevent the outbreak of World War 2, as it lacked the power and support to enforce its decisions.
The Trigger:
The immediate trigger for World War 2 was the invasion of Poland by Germany in September 1939. Britain and France, which had guaranteed Poland's independence, declared war on Germany in response to this invasion. This marked the beginning of World War 2.